The main app icon is showing with a black border/background in the Task Bar. The icon I am providing does not have any gaps in it, it is completely filling the image. How can I make the icon be full screen and not have this black border? Please see screenshot for more details.
Have you provided all scales for that icon?
Yes. On second look it seems like all app icons from the Windows Store have this kind of border/tile. Must be part of the operating system.
Could be. In such case you have to use Tile API to change that.
Is that part of Unity?
Yes, https://docs.unity3d.com/ScriptReference/WSA.Tile.html
Though, full feature set is available using Microsofts live tile API.