RtpTransceiver: Inject packet callbacks at channel construction time.

Update the RtpTransceiver and Channel implementations to inject packet
event callbacks directly during the construction of media channels. This
replaces the previous pattern of assigning these callbacks via setter
methods after the channel was already instantiated.

Bug: webrtc:42222804
Change-Id: Ia0c76f78b8698156de99301c51a272a45d941b47
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/462840
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47381}