AudioPolicy: add synchronous unregister method

Add a method to synchronously unregister an AudioPolicy. This method
   also flushes the AudioRecord and AudioTrack instances that
   were associated with any AudioMix created by this policy.

Bug: 119057550
Test: see bug
Change-Id: Ia58ccdaf8a50580bcdd49aa2f6e204f0f6c7f814