Camera2: Skeleton for output frame processing, plus face detect

- Plumbing for processing output metadata frames from the HAL
- Support for passing face detection metadata from said frames
  to the application.
- Switch calls on ICameraClient interface to use separate mutex to
  avoid deadlock scenarios with messages being communicated from the
  HAL to the camera user while calls from the user to the service are
  active.

Bug: 6243944

Change-Id: Id4cf821d9c5c3c0069be4c0f669874b6ff0d1ecd