audio: Restructure A2DP active device change

Make active device change asynchronous.
Restructure A2DP active device change API
for clarity and speed.
Merge active device and device config change
handlers.
Fix bug for a new A2DP device connection with
handleBluetoothA2dpActiveDeviceChange.

Bug: 122944517
Test: still needs Bt side using new interface
Change-Id: Ia2fa6b2c98ed4452880e8abca4a069b904f8cc68