Add SSRC tracking and receive sink clearing scaffolding

Add scaffolding changes to prepare the channel layer for direct packet
delivery on the network thread. This brings architectural updates to
allow upcoming changes for routing packets on the network thread.

- Add ClearReceiveSinks_n, SetReceiveSsrcs_n, and GetUnsignaledSsrcs
  methods to MediaReceiveChannelInterface.
- Update BaseChannel methods (DisconnectFromRtpTransport_n,
  RegisterRtpDemuxerSink_w, and UpdateRemoteStreams_w) to pass active
  and removed SSRCs down to the receive channels.
- Implement extraction and tracking of removed SSRCs, including
  unsignaled streams, during remote stream updates.

Bug: webrtc:42222117
Change-Id: I6f28468dbf1dcf270ee360d9d44b8c0256e638b4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/474580
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47776}