camera: Add metadata-based ASD

 * New cameras use the metadata callback to report the
   detected scene mode. Add support for this.
 * The asdModes array must be set for this to work, which
   should contain the ordered list of possible scenes,
   corresponding to the camera_asd_values array.

Change-Id: I07279ae30a4cbedef02bec702252eb7e2abacd4a