Require HasChannel calls to be called on the signaling thread

Update stat collection to not call HasChannel or receivers() from the
worker thread, grab state earlier in the gatherer object.

Also,
- Update RtpTransceiver::SetRtpTransport to return early if no channel
  exists (consistent with previous HasChannel() check).
- Update FakePeerConnectionForStats to support DTLS transport lookups
  and property assignments (transport_name) to improve compatibility
  with production code gathering logic.

Bug: webrtc:475126742
Change-Id: If31fbd25d5bcad911e9ed1866b40c41481b81cd3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/468900
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47607}