MediaSession2: Workaround javadoc issue with hidden generic super class

This includes two changes.
  - Change all methods in MediaSession2.BuilderBase signatures from
    public to package private.
  - Subclasses of MediaSession2.BuilderBase (MediaSession2.Builder,
    MediaLibraryService2.MediaLibrarySessionBuilder) now inherit all
    methods from its super class and change method signatures to public

Changes are needed to workaround and fix following issues.
  - Hidden super class's methods aren't shown in the Javadoc and 3rd
    party apps cannot use them.
  - Public generic methods in the super classes are shown in the Javadoc
    with the generic types (e.g. 'U setSessionCallback()' instead of
    'Builder setSessionCallback()')
  - Stub classes for tests are automatically generated, but it somehow
    fails to generate stub for the BuilderBase (seems like a bug)

Test: 'm update-api' and checked generated javadoc
Change-Id: I6f635624ef806578ab79d080424f11cb637d7984