Migrate ringtone playback to SystemUI.

Introduce IRingtonePlayer, which handles playback for both Ringtone
objects and Notifications. SystemUI now hosts this player, which it
registers with AudioService. It also keeps MediaPlayer instances
warm, and cleans them up after stop() or Binder death.

Move both Ringtone and NotificationManagerService to play back audio
through this new interface.

Bug: 6376128, 6350773
Change-Id: I1dcb86d16ee3c4f07cdb2248d33dcff4ead3609a