Revise new public API for ratings in RemoteControlClient

Refactor RemoteControlClient.MetadataEditor to move functionality
 in new abstract class MediaMetadataEditor, so it can be also
 used on the "display" side of the RemoteControl functionality.
Compatibility of these changes has been tested against
 existing Play applications which use the RemoteControlClient API.

Move the new constants and method definitions related to the
 ratings feature to MediaMetadataEditor. This changes the
 yet-to-be-published ratings API, so this doesn't break
 compatibility.
Rating feature is handled by the new Rating class with a set
 of constructors that handle the different rating models.

Bug 8440498

Change-Id: I9b6dc2204bfc48594bad5cd6449d357f8a485da0