audioflinger: Set power hints for audio activity

 * Send a hint to the power manager when audio starts or stops.
 * This lets us behave in a smart way when streams are active-
   ie, don't mess with timer_rate while a2dp is active, which
   can cause skipping. Same deal for USB audio.

Change-Id: Ic1cf3f96afb44a8a5c4af4503cf7b30eed491910
audioflinger: Power hints for audio activity

 * Take 2, now with 100% less crashes!

Change-Id: I48fb0d2e3cfc64f46d99f177b4cf3fe842d06df4