A2dpSrc: Added seperate synchronization for mAudioManager calls.

- Deadlock between btif thread and A2dpservice thread due to Setparams
  call has been blocked on lock mVariableLock. It has been happend in
  below way.

  When START done, A2dp is updating MM as TWSchannel mode would be “mono”
  through setparams call by acquiring mVariableLock in A2dpService thread.
  Then BTM_BLI_ACL_DOWN_EVT comes from stack is updating to App in btif
  thread and same is waiting for the lock mVariableLock in messageFromNative()
  API, which is the reason when MM sent disable_snd_device for bt-a2dp which
  calls audio_stream_stop_qti api from BT HAL, but not sent to btif thread
  because of btif thread stuck as mentioned above, which resulted in TimeCheck
  timeout.

- Fixed the above deadlock in below way.

  Synchronize mAudioManager calls on seperate Lock, instead of
  mVariableLock. So, that it won't block btif thread for callbacks
  processes.

- Also taken care synchronization of mBtAvrcpLock, and mBtA2dpLock

CRs-Fixed: 2663821
Change-Id: Ia6247dcfb4964a8f6ccc67ec06407fc0500ad8b0