Prepare comments and annotations for public FabricatedOverlay API

This patch adds the following items.
* FabricatedOverlay class comments.
* add a FabricatedOverlay contructor
* add a public method `setTargetOverlayable`
* add setResourceValue APIs for
  * int
  * String
  * ParcelFileDescriptor
* comments and annotations for methods in FabricatedOverlay.
  * such as nullability

Bug: 205919743
Test: atest \
        OverlayHostTests \
        OverlayDeviceTests \
        SelfTargetingOverlayDeviceTests \
        OverlayRemountedTest \
        FrameworksServicesTests:com.android.server.om \
        CtsContentTestCases:android.content.om.cts \
        idmap2_tests
Change-Id: I689f5882fc3e1fc74a343106ef310f23a83990cf