A2DP: Simplify debug log mechanism

* Override log() method in StateMachine to log messages gated by DBG
  flag
* Replace all usages of "if (DBG) Log.d(TAG, msg)" with log(msg)
* Call StateMachine.setDbg(DBG) in the constructor to enable more
  logging for StateMachine
* Add null check to getCurrentMessages() in logging to avoid unintended
  NPE

Bug: 73899051
Test: connect to and disconnect from A2DP devices
Change-Id: Id89f3ba99c97c62dca74fa3901dc3c6f2c352fc1