Use Unified Plan transceivers in FakePeerConnectionForStats

FakePeerConnectionForStats put all senders and receivers of a media type
on one Plan B transceiver, so the stats collectors were tested with a
setup that Unified Plan never produces.

- Add AddRtpTransceiver(), which creates a Unified Plan transceiver with
  a given sender and receiver. A missing sender or receiver is replaced
  by a NiceMock. The default receiver has a track without an ID, because
  the legacy collector dereferences receiver tracks and an empty ID
  keeps that track out of the reports.
- AddVoiceChannel() and AddVideoChannel() put the channel on the
  transceiver with the requested mid, else on the first transceiver of
  the media type that has no mid yet, else on a new transceiver.
- GetSenders() and GetReceivers() return the sender and the receiver of
  each transceiver that is not stopped.
- Remove AddSender(), RemoveSender(), AddReceiver(), RemoveReceiver(),
  the Plan B transceiver helpers and the PLAN_B_ONLY markers.

The tests add each sender and receiver with AddRtpTransceiver(). Where a
sender and a receiver belong to the same m= section, they share a
transceiver. The suppressions of Plan B deprecation warnings are removed
from these tests. The Plan B test file keeps its own Plan B fake and is
unchanged.

Bug: webrtc:470300031
Change-Id: I77e7584e6284ed13ff1134dc10a314e9f4c045b2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/508481
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#49061}