[M155] Keep transceiver MID in sync with its channel

Original change's description:
> 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}

(cherry picked from commit 7a25e1c85a0af15cae8a2d55bf02e5610bf1af53)

Bug: chromium:567160162
Fixed: chromium:569045198
Change-Id: Ic9f29b66a8f9f181fd414009f5c1e5cebfa9ca54
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/507140
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/branch-heads/8059@{#4}
Cr-Branched-From: 8d208e76fb1ad2b92d557f752e9d1d7d49059efa-refs/heads/main@{#48593}