snap: Add constrained longshot mode

 * On the OP3, we can only use Longshot in a single scene mode without
   conflicting with other built-in postprocessing features which cannot
   be disabled. Add support for this. When a scene specified in the
   longshot-scenemodes list is active, continuous shot will be enabled.

Change-Id: I79878e5ac918e907ddc5b3ca168e49f4e06656c3
2 files changed