Prepare comments and annotations for OverlayInfo & OverlayIdentifier public API

This patch doesn't change any logic or data type. It only prepare the
comments for public API in OverlayInfo and OverlayIdentifier.

In both of OverlayInfo and OverlayIdentifier
* Add class comments for self-targeting
* Add comments and annotations for the APIs will be public
  * such as nullability
* Add @hide for the APIs not to public
* Separate the comments for OverlayManagerService with HTML comments.

OverlayInfo
* Add @see relevant APIs.

OverlayIdentifier
* change protected to be /* package-private */
* Make the class definition to be final because of parcelable.

Bug: 205919743

Test: make update-api ; make
Test: make online-sdk-docs offline-sdk-docs
Test: atest \
        OverlayHostTests \
        OverlayDeviceTests \
        SelfTargetingOverlayDeviceTests \
        OverlayRemountedTest \
        FrameworksServicesTests:com.android.server.om \
        CtsContentTestCases:android.content.om.cts \
        CtsContentTestCases:android.content.res.loader.cts \
        idmap2_tests
Change-Id: I5dc4dca33b75bdd382e817803927fa33bdb84667