Allow injecting MediaSendChannel into RtpSender at construction time.

Slightly refactor `RtpSender` and its subclasses (`AudioRtpSender`,
`VideoRtpSender`) to accept, but not yet require, the
`MediaSendChannelInterface` at construction time. Previously, the
channel was always set via `SetMediaChannel` immediately after creation,
which is more complex but can be avoided.

This change allows for stricter threading assertions within `RtpSender`.
Several methods accessing `media_channel_` are now guarded with
`RTC_DCHECK_RUN_ON(worker_thread_)` to clarify thread safety.

Updates `RtpTransmissionManager` for PlanB (UP requires more work), to
pass the appropriate voice or video media channel when creating senders,
and adjusts `RtpSenderReceiverTest` to match the new constructor
signatures.

Bug: none
Change-Id: Iad3691d1dbccded53716066bb199d6e1f0e0e40a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/431240
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46405}