Camera2: Prevent autofocus when video snapshot is in progress

During video recording if screen is tapped repeatedly, autofocus is
started while the snapshot is taken. This would either result in an
out of focus snapshot which would also disturb the focus in the recorded
video or crash the sensors that can't handle autofocus during snapshot.

Change-Id: If2763094f5ab7536e4409469276de54580e803c8