Limit RemoteControlClient generation ID changes

At the beginning of each song, the Music app re-registers its
 media button event and requests audio focus, which causes
 the reevaluation of each corresponding stack of clients. Each
 reevaluation is accompanied by the incrementation of the
 RemoteControlClient generation ID, which causes RemoteController
 to issue a notification that the client has changed. The lockscreen
 correctly interprets this as a reason to dump the current
 RemoteControlClient data (including the artwork) because it will
 receive the new data if new one is available. This is what causes
 the "flashing" of the wallpaper on the lockscreen: for an instant,
 no client data is available.
The fix consists in not causing the client generation ID to be
 incremented when registrations don't cause any change in the
 RemoteControlClient stack. Even though Music re-registers everything,
 nothing has changed: it still is the current media button receiver,
 and it still has the same RemoteControlClient.

Bug 11307382

Change-Id: I4d2404b571e88aeedb0eca6bd19d39f7ec4fc8b1