Allow injecting media channel into VideoRtpReceiver

This aligns VideoRtpReceiver and AudioRtpReceiver by allowing the
media channel to be passed directly during construction. This
dependency injection is necessary preparation for creating senders
and receivers from within the RtpTransceiver class.

Also create parity between the audio and video receiver
constructors by removing the explicit `is_unified_plan` boolean from
`AudioRtpReceiver`.

Key changes:
* Update `VideoRtpReceiver` to accept an optional
  VideoMediaReceiveChannelInterface` in the constructor.
* Refactor `AudioRtpReceiver` constructors to not have the
  `is_unified_plan` argument in constructors that are only used with
  unified plan. Leave one dedicated constructor for planb.
* Update call sites in `RtpTransmissionManager` and unit tests.

Bug: webrtc:42222804
Change-Id: Ifa82f5ac4087b048f5e6d78284a4e79678c8e3b6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/433841
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46499}