Camera3: Allow multiple process_capture_result calls per request

To simplify HAL implementation, allow the results for a single
capture request to be sent from the HAL across multiple calls
to process_capture_request.

This requires that the HAL must call notify() with the SHUTTER message
to indicate start of exposure time, before the first call to
process_capture_result for a given frame.

Bug: 8565103

Change-Id: I6a61449725f98af88769fafa07736848a226dad2