Improve internal state synchronization of the A2dpStateMachine

 * Don't block entering Disconnected state
   If the mCurrentDevice or mTargetDevice or mIncomingDevice
   is not null, print an error message and reset the state
 * Add consistency checks when entering each state: Disconnected,
   Pending, Connected
 * Add a missing "synchronized" statement

Bug: 62164051
Test: Manual
Change-Id: I5856a88e58b42aa55d9075aa507bfa706ea52512
(cherry picked from commit 459978de)