AudioService: make BT A2DP API async

Handle the BT A2DP methods for connection / disconnection /
configuration change asynchronous by handling them on
AudioService's message queue.

Bug: 162448412
Test: atest AudioDeviceBrokerTest
Change-Id: I36e6fea87110e3901a1dfb76ef8c36b2b1a732d0
4 files changed