Fixes due to pause video and take photo while recording video buttons being gone instead of invisible.

Set faces_detected to null on ui thread if camera is closed; move accessibility code to new reportFaces().

Move post start() code to videoRecordingStarted; some of this code should only be done if we didn't throw an exception.

Better UI performance when switching from photo to video mode - always reopen the camera, as that can be done on background thread.

Save base expo images with suffix _0 etc instead of _EXP0 etc, so that Google Photos will group them.