Enforce consistent network thread usage in Call::OnSentPacket

Remove the `sent_packet_sequence_checker_` checker in Call and use the
less ambiguous network_thread_ instead. Update OnSentPacket with a
strict requirement to run on the network thread. Previously, a detached
sequence checker allowed inconsistent threading behavior between
production and test environments, which obscured the actual threading
model and complicated maintenance of outgoing traffic logic.

Update tests to consistently follow this for better alignment with the
production code.

Move the responsibility of doing a thread hop for `DeliverRtcpPacket`
out of PeerConnection and into Call. This thread hop is a current
implementation detail for the Call class that we'd like to remove.

Key modifications:
* Update transport wrappers to ensure sent packet notifications are
  dispatched on the correct thread, hopping if necessary.
* Add internal thread-hopping to DeliverRtcpPacket to bridge from the
  network thread to the worker thread.
* Standardize the use of Thread in test configurations to
  simulate realistic network thread behavior.

Bug: webrtc:42222117
Change-Id: Ibcdfe0c3b5d0b20ff47b536929e8100fb70fd2f2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/469080
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47596}