The type or namespace name `AndroidDevice' could not be found. Are you missing an assembly reference?

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The type or namespace name `AndroidDevice’ could not be found. Are you missing an assembly reference?
Error building Player because scripts had compiler errors

You forgot add the assembly reference where that class live.
if you are on Visual studio click in the AndroidDevice class and press " ctrl + . " and you will get some suggestions.
See if there exist the assembly where that class live and add the reference.

// Copyright 2015 Google Inc. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the “License”);
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an “AS IS” BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#if !UNITY_EDITOR
#if UNITY_ANDROID
#define ANDROID_DEVICE
#elif UNITY_IPHONE
#define IPHONE_DEVICE
#endif
#endif

using UnityEngine;
using System.Collections.Generic;
using System;

/// @cond
namespace Gvr.Internal {
// Represents a vr device that this plugin interacts with.
public abstract class BaseVRDevice {
private static BaseVRDevice device = null;

protected BaseVRDevice() {
  Profile = GvrProfile.Default.Clone();
}

public GvrProfile Profile { get; protected set; }

public abstract void Init();

public abstract void SetUILayerEnabled(bool enabled);
public abstract void SetVRModeEnabled(bool enabled);
public abstract void SetDistortionCorrectionEnabled(bool enabled);

public abstract void SetSettingsButtonEnabled(bool enabled);
public abstract void SetAlignmentMarkerEnabled(bool enabled);
public abstract void SetVRBackButtonEnabled(bool enabled);
public abstract void SetShowVrBackButtonOnlyInVR(bool only);

public abstract void SetNeckModelScale(float scale);
public abstract void SetAutoDriftCorrectionEnabled(bool enabled);
public abstract void SetElectronicDisplayStabilizationEnabled(bool enabled);

public virtual bool SupportsNativeDistortionCorrection(List<string> diagnostics) {
  return true;
}

public virtual bool RequiresNativeDistortionCorrection() {
  return leftEyeOrientation != 0 || rightEyeOrientation != 0;
}

public virtual bool SupportsNativeUILayer(List<string> diagnostics) {
  return true;
}

public virtual bool ShouldRecreateStereoScreen(int curWidth, int curHeight) {
  return this.RequiresNativeDistortionCorrection()
         && (curWidth != (int)recommendedTextureSize[0]
             || curHeight != (int)recommendedTextureSize[1]);
}

public virtual RenderTexture CreateStereoScreen() {
  float scale = GvrViewer.Instance.StereoScreenScale;
  int width = Mathf.RoundToInt(Screen.width * scale);
  int height = Mathf.RoundToInt(Screen.height * scale);
  if (this.RequiresNativeDistortionCorrection()) {
    width = (int)recommendedTextureSize[0];
    height = (int)recommendedTextureSize[1];
  }
  //Debug.Log("Creating new default stereo screen texture "
  //    + width+ "x" + height + ".");
  var rt = new RenderTexture(width, height, 24, RenderTextureFormat.Default);
  rt.anisoLevel = 0;
  rt.antiAliasing = Mathf.Max(QualitySettings.antiAliasing, 1);
  return rt;
}

// Returns true if the URI was set as the device profile, else false.  A default URI
// is only accepted if the user has not scanned a QR code already.
public virtual bool SetDefaultDeviceProfile(Uri uri) {
  return false;
}

public virtual void ShowSettingsDialog() {
  // Do nothing.
}

public Pose3D GetHeadPose() {
  return this.headPose;
}
protected MutablePose3D headPose = new MutablePose3D();

public Pose3D GetEyePose(GvrViewer.Eye eye) {
  switch(eye) {
    case GvrViewer.Eye.Left:
      return leftEyePose;
    case GvrViewer.Eye.Right:
      return rightEyePose;
    default:
      return null;
  }
}
protected MutablePose3D leftEyePose = new MutablePose3D();
protected MutablePose3D rightEyePose = new MutablePose3D();

public Matrix4x4 GetProjection(GvrViewer.Eye eye,
                               GvrViewer.Distortion distortion = GvrViewer.Distortion.Distorted) {
  switch(eye) {
    case GvrViewer.Eye.Left:
      return distortion == GvrViewer.Distortion.Distorted ?
          leftEyeDistortedProjection : leftEyeUndistortedProjection;
    case GvrViewer.Eye.Right:
      return distortion == GvrViewer.Distortion.Distorted ?
          rightEyeDistortedProjection : rightEyeUndistortedProjection;
    default:
      return Matrix4x4.identity;
  }
}
protected Matrix4x4 leftEyeDistortedProjection;
protected Matrix4x4 rightEyeDistortedProjection;
protected Matrix4x4 leftEyeUndistortedProjection;
protected Matrix4x4 rightEyeUndistortedProjection;

public Rect GetViewport(GvrViewer.Eye eye,
                        GvrViewer.Distortion distortion = GvrViewer.Distortion.Distorted) {
  switch(eye) {
    case GvrViewer.Eye.Left:
      return distortion == GvrViewer.Distortion.Distorted ?
          leftEyeDistortedViewport : leftEyeUndistortedViewport;
    case GvrViewer.Eye.Right:
      return distortion == GvrViewer.Distortion.Distorted ?
          rightEyeDistortedViewport : rightEyeUndistortedViewport;
    default:
      return new Rect();
  }
}
protected Rect leftEyeDistortedViewport;
protected Rect rightEyeDistortedViewport;
protected Rect leftEyeUndistortedViewport;
protected Rect rightEyeUndistortedViewport;

protected Vector2 recommendedTextureSize;
protected int leftEyeOrientation;
protected int rightEyeOrientation;

public bool triggered;
public bool tilted;
public bool profileChanged;
public bool backButtonPressed;

public abstract void UpdateState();

public abstract void UpdateScreenData();

public abstract void Recenter();

public abstract void PostRender(RenderTexture stereoScreen);

public virtual void OnPause(bool pause) {
  if (!pause) {
    UpdateScreenData();
  }
}

public virtual void OnFocus(bool focus) {
  // Do nothing.
}

public virtual void OnLevelLoaded(int level) {
  // Do nothing.
}

public virtual void OnApplicationQuit() {
  // Do nothing.
}

public virtual void Destroy() {
  if (device == this) {
    device = null;
  }
}

// Helper functions.
protected void ComputeEyesFromProfile() {
  // Compute left eye matrices from screen and device params
  Matrix4x4 leftEyeView = Matrix4x4.identity;
  leftEyeView[0, 3] = -Profile.viewer.lenses.separation / 2;
  leftEyePose.Set(leftEyeView);

  float[] rect = new float[4];
  Profile.GetLeftEyeVisibleTanAngles(rect);
  leftEyeDistortedProjection = MakeProjection(rect[0], rect[1], rect[2], rect[3], 1, 1000);
  Profile.GetLeftEyeNoLensTanAngles(rect);
  leftEyeUndistortedProjection = MakeProjection(rect[0], rect[1], rect[2], rect[3], 1, 1000);

  leftEyeUndistortedViewport = Profile.GetLeftEyeVisibleScreenRect(rect);
  leftEyeDistortedViewport = leftEyeUndistortedViewport;

  // Right eye matrices same as left ones but for some sign flippage.
  Matrix4x4 rightEyeView = leftEyeView;
  rightEyeView[0, 3] *= -1;
  rightEyePose.Set(rightEyeView);

  rightEyeDistortedProjection = leftEyeDistortedProjection;
  rightEyeDistortedProjection[0, 2] *= -1;
  rightEyeUndistortedProjection = leftEyeUndistortedProjection;
  rightEyeUndistortedProjection[0, 2] *= -1;

  rightEyeUndistortedViewport = leftEyeUndistortedViewport;
  rightEyeUndistortedViewport.x = 1 - rightEyeUndistortedViewport.xMax;
  rightEyeDistortedViewport = rightEyeUndistortedViewport;

  float width = Screen.width * (leftEyeUndistortedViewport.width+rightEyeDistortedViewport.width);
  float height = Screen.height * Mathf.Max(leftEyeUndistortedViewport.height,
                                           rightEyeUndistortedViewport.height);
  recommendedTextureSize = new Vector2(width, height);
}

private static Matrix4x4 MakeProjection(float l, float t, float r, float b, float n, float f) {
  Matrix4x4 m = Matrix4x4.zero;
  m[0, 0] = 2 * n / (r - l);
  m[1, 1] = 2 * n / (t - b);
  m[0, 2] = (r + l) / (r - l);
  m[1, 2] = (t + b) / (t - b);
  m[2, 2] = (n + f) / (n - f);
  m[2, 3] = 2 * n * f / (n - f);
  m[3, 2] = -1;
  return m;
}

public static BaseVRDevice GetDevice() {
  if (device == null) {

#if UNITY_EDITOR
device = new EditorDevice();
#elif ANDROID_DEVICE
device = new AndroidDevice();
#elif IPHONE_DEVICE
device = new iOSDevice();
#else
throw new InvalidOperationException(“Unsupported device.”);
#endif
}
return device;
}
}
}
/// @endcond

//This is code in which #elif ANDROID_DEVICE
// device = new AndroidDevice(); line is giving error

Ok Thanks i got