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