Problem when add android plugin

I add an android plugin in my project, and create a subfolder to hold it. when /lib and /res. it not work, and /StagingArea not contains the /res. But when i move /res fold under /Android, it works fine. what is the problem? I have create project.properties and sub AndroidManifest.xml under my subfolder.

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It should work fine.
Does your project.properties contain android.library=true?
Make sure to read Unity - Manual: Android Library Projects and Android Archive plug-ins

i use .jar and i truely contain android.library=true