Simplify RtpTransceiver and BaseChannel construction threading

Perform media channel creation and `BaseChannel` construction
synchronously on the signaling thread rather than deferring them to the
worker thread. This change avoids worker thread tasks when possible. and
simplifies the asynchronous initialization process.

Bug: webrtc:42224170
Change-Id: If5d3d68ee334609bce243b06fb0f7be352bb4bda
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/475140
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47908}