HFP: Dump state machine message verbatim

* Override StateMachine.getLogRecString(Message) method so that state
  machine messages are dumped verbatim into dumpsys output
* Use StringBuilder to build strings in getLogRecString(Message) and in
  HeadsetStackEvent.toString() to avoid costly String copy
* Optimize HeadsetStackEvent.getTypeString() to return static string
  directly
* Create HeadsetMessageObject base class to define toString() and
  buildString() interfaces
* Add buildString() methods to:
  - HeadsetDeviceState
  - HeadsetCallState
  - HeadsetClccResponse
  - HeadsetVendorSpecificResultCode
  - HeadsetStackEvent
* Split above classes into individual files

Bug: 72896133
Test: make, adb shell dumpsys bluetooth_manager
Change-Id: I319af5103168d41adf75feb2f0adad88c1c41e5f