Playback notification: only report state changes

When an SL ES player plays back data received from a
  buffer queue, the started state is reported on each
  buffer refill. Only send AudioService updates on
  the player state when the state has changed.

Test: enable logging in PlaybackActivityMonitor and play audio in Chrome, observe no log spam
Bug: 30258418

Change-Id: Id962503edbdea93cb6221e3c50b8d440f388b831