Bug 1817724 - In VideoCaptureAvFoundation use MediaEvent to handle frames. r=webrtc-reviewers,ng

Hypothetically if the frame callback is doing something leading to crashes, this
patch should at least affect those symptoms, giving us more clues to where the
problem lies.

This patch changes away from calling the CamerasParent callback directly on the
frame queue, to notifying a MediaEvent owned by the VideoCaptureAdapter. The
frames are then handled on the video capture thread.

Differential Revision: https://phabricator.services.mozilla.com/D171962