Fix bugs uncovered by HeadsetClientStateMachineTest

* State change broadcast should always happen after actual state
  transitions. Therefore, state change broadcast should only happen in
  enter() and exit() methods of states, except when no actual state
  transition is involved. Otherwise, there is a race condition between
  state change broadcast and actual state transition. If user of state
  machine sends message for new state indicated by the broadcast, the
  state machine may still be in previous state and hence result is
  undefined depending on timing condition.
* This CL fixes this issue by moving all state
  change broadcasts to enter() and exit() methods of individual states
  and log previous state in exit()
* Also, messages that are supposed to be executed in the next state
  should be sent using deferMessage() instead of sending message after
  transitionTo() is called
* Remove some unused variables from HeadsetClientStateMachine
* Unified HeadsetClientStateMachine.start()/stop() into setUp() and
  tearDown() methods in HeadsetClientStateMachineTest
* Fixed testIncomingPriorityAccept to keep up with recent change:
  I6c5a7ee9eb4e10f5f649794546acde5ec4b297aa
* Use explicit intent matcher instead of matching any intents for the
  above fixed test

Bug: 68722902
Test: build, run unit tests
Change-Id: Iccbaec9df0abfc9136e5b86b450151b13b45cfa7