Audio policy: volume computation improvement on A2DP for notifications

When playing STRATEGY_SONIFICATION* streams, or system sounds or
  enforced sounds on A2DP during media playback, ensure the
  difference between the media volume and the notification
  is no more than 12dB

Bug 28288132

Change-Id: If5cb7979dd76a0d624e8aeff9126ff56bfd7366b