Single threaded RtpTransceiver construction

Move the creation of media content channels and the RTP sender/receiver
directly to the signaling thread within the RtpTransceiver constructor.
This is now possible following some previous updates to the other
classes.

Previously, these operations were delegated to the worker thread using a
ScopedOperationsBatcher (`worker_tasks`). By executing them
synchronously on the signaling thread, this change simplifies the
initialization process and entirely removes the need for task batching
during transceiver construction.

* Update `CreateAndAddTransceiver` and `AssociateTransceiver` methods
  to remove the `worker_tasks` parameter across all call sites.
* Adapts related `RtpTransceiver` tests.

Bug: webrtc:42224170
Change-Id: I2497af614d0d36520ac9793d6c59e00370385eea
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/475100
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47829}