Guard buffer_ and handle stream restart in VideoReceiveStream2

Store decode_sync_ pointer in VideoReceiveStream2 and guard buffer_ with
worker_sequence_checker_. Recreate buffer_ in Start() if it has been
stopped.

Recreating buffer_ in Start() when it has been stopped allows
VideoReceiveStream2 to be stopped and restarted cleanly. Calling Stop()
marks the underlying VideoStreamBufferController as stopped, which halts
frame decode scheduling; recreating the controller upon restart resets
it to an active state, clears stale frame and timer state from the
previous run, and re-registers a fresh decode scheduler with
DecodeSynchronizer.

Bug: none
Change-Id: Ic57ebaf991e1b047665524839fc9367801428afe
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/496240
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48409}