Keep transceiver MID in sync with its channel

When applying a local description, a transceiver could be matched to an
m= section by its index even if it was already associated with a
different MID. The channel of the transceiver keeps using the MID it was
created for, so the two could get out of sync.

- Only match transceivers that are not yet associated by m= section
  index.
- Route transport changes to a transceiver based on the MID of its
  channel.

Bug: chromium:567160162
Change-Id: Ic9f29b66a8f9f181fd414009f5c1e5cebfa9ca54
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/506421
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48768}