Expose a way to set a PendingIntent for restarting playback

This is to allow apps to set a PendingIntent to restart playback. This
is not persisted across reboots but will allow music to start playing
again for the life of the system. Only the most recent priority app with
a PI set will be cached. This also deprecates methods in AudioManager
that do this and unhides the unregister method in MediaSessionManager.

Change-Id: I66fbf5856333468d8cb8a3022809778ba00d426e