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)

This is a port of I367bc45729ce2e858fea972f35e881a755c6021a from cm-11

Change-Id: I9a065f951f628d1c550acbac18493b9ad3853d03