Move payload type demuxing management to RtpTransport

Shift the responsibility for enabling and disabling payload type
demuxing from the Channel and Transceiver layers to the RtpTransport and
RtpDemuxer layer. This centralizes demuxing logic within the transport
layer.

Moving this logic to RtpTransport allows the SdpOfferAnswerHandler to
apply state changes directly to the transport and not need to go through
the Channel classes.

Key modifications:
- Remove SetPayloadTypeDemuxingEnabled from BaseChannel,
  ChannelInterface, and RtpTransceiver.
- Implement SetActivePayloadTypeDemuxing in RtpTransport to manage
  demuxer criteria filtering and coordinate with RtpDemuxer.
- Update SdpOfferAnswerHandler to apply demuxing state changes to
  RtpTransport instances directly.
- Ensure ResetUnsignaledRecvStream executes on the worker thread
  after applying transport state, to clear legacy streams when
  demuxing is disabled.

Bug: webrtc:42222117
Change-Id: If02194417dc8e4c3dcf5fda5e969aa81116004e1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/456720
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47318}