Raise A2DP threat priority to avoid music breaks

* Ensure all a2dp audiopath threads are configured to high
  priority when actively streaming.
* For now set bt hc worker thread always to URGENT_AUDIO
  until new thread api can adjust thread priority dynamically.

Bug: 17520043
Change-Id: I10e314085329278bcfcc4e8bc5ce899b03d22e2b