A2DP: Dump state machine message verbatim

* Override StateMachine.getLogRecString(Message) method so that state
  machine messages are dumped verbatim into dumpsys output
* A2dpStackEvent will be dumped automatically via the toString() method

Bug: 72102258
Test: adb shell dumpsys bluetooth_manager
Change-Id: I12dedb1a6ee790572072e589e7d480198043b2a7