Add RTC_GUARDED_BY to RtpSender::media_channel_

Annotate the `media_channel_` member in `RtpSenderBase` with
`RTC_GUARDED_BY(worker_thread_)`.

Refactor usage in methods like `GetParameters`, `SetParameters`,
`SetAudioSend`, and `SetVideoSend` accordingly.

Additionally:
* Annotate `last_transaction_id_` and `observer_` with
  `RTC_GUARDED_BY(signaling_thread_)`.
* Convert `CheckCodecParameters` from a virtual method to a private
  helper function restricted to the worker thread.
* Remove the unsafe assignment of `media_channel_` to `nullptr` in
  `Stop()`, avoiding a write violation from the signaling thread.

Bug: none
Change-Id: I748cce8d073ce398d653fca98b39838cf1c2cf33
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/434580
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46503}