cameraservice: Resolve message vs start/stop preview races

Move the mutex in start/stopPreview to include the metadata message
management. Fast start/stop sequences could (and would) eventually
cause the messages to be enabled/disabled out of sequence (or while
the camera wasn't ready for it)

Fixes BACON-173

Change-Id: I367bc45729ce2e858fea972f35e881a755c6021a