camera: Clear any leftover scene modes when entering video mode

 * HDR scene mode might still be set when entering video mode, since
   we don't reset the hardware during the transition anymore. This
   can be incredibly bad on some devices such as JF, who get wildly
   confused when one tries to do a video snapshot while recording.
 * Fixes the crash on JF, possibly other issues as well.

Change-Id: I635f65742ab1f73cfeeec0890d31208502b146f5