Reduced the audio effort while changing the A2DP active device

* Functions to change the active device must be exclusive of each other,
  and is separated from the mStateMachine synchronized.
* Move the mAudioManager calls out of the mStateMachine synchronized,
  and it breaks the deadlock even if the AudioManager invokes A2DP APIs
  in the same time.

Bug: 132035154
Bug: 128962562
Test: TH and switch A2DP device manually
Change-Id: Ib55f5dc51dfe0354b106bdd5bb7cee59c9e149e1