Move audio and video packet demuxing to the network thread

Move RTP packet demuxing in Call to the network thread. By resolving the
correct receiver sink earlier in the pipeline, we encapsulate the
asynchronous demuxing process while maintaining the autonomy of
individual receive streams.

To safely handle the cross-thread delivery of packets—where a sink might
be resolved on the network thread but destroyed before the worker thread
can process the packet—an `RtpSinkValidator` interface is introduced.
`Call` implements this validator to track active sinks and verifies
their existence prior to packet dispatch, preventing use-after-free
bugs.

Additionally, a `DisconnectFromNetworkThread` method is added to
ensure safe teardown during connection closure.

Bug: webrtc:42222117
Change-Id: Ieddee38399d5d8098ae9e03a1a8a6ed3f86783d6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/472240
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47733}