Prevent crashing when disabling MCU during dream wakeup

AmbientMode attempting to send message to DozeHardware no longer results in an
uncaught IllegalStateException.  The exception was crashing Android Wear when
DozeHardware had already been released by PowerManagementService at time of
AmbientMode's message.

Change-Id: I5956a453802b5b764e88638f6b6d899cbc94cc8d