Get bluetooth A2DP status directly from BT

MediaRourter uses main thread for internal operations, and ANR could
happens if AudioService is not running when MediaRouter tried to get
BT A2DP status. This CL avoids such ANR by getting the info using
BroadcastReceiver insteead of AudioService.

Bug: 170327593
Test: manually
Change-Id: If9eba19481fceb0c86ff0117e8a604b64d7ebc49