camera: Hold 25 msec between preview cycles on restart

If doing a preview restart with an active listener, give said
listener a bit of time to clean up before starting up the preview
again. This should be sufficient to prevent shutdown/start race
conditions during externally triggered restarts.

Change-Id: Id4a722c472eeffb50a332b0dabc29d9e23062883
Merge branch 'cm' into HEAD