Refactor RtpTransceiver::channel() to be private

This CL moves the RtpTransceiver::channel() accessor to the private section
to better encapsulate the internal channel object. Public wrapper methods
are added to RtpTransceiver for common ChannelInterface operations, and
specific accessors for media channels (e.g., media_send_channel(),
voice_media_receive_channel()) are provided. All callers in the codebase
are updated to use these new public interfaces.

Bug: webrtc:42224170
Change-Id: Ia67c28cf3eaf06a5b4f3acc5c45e38a28978d519
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/440688
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46656}