Provide thread-safe channel access during transport changes

Introduce a mechanism to safely access active channels on the network
thread during transport-related operations. Previously,
OnTransportChanged traversed transceivers to find the associated
channel, which risked accessing signaling-thread-guarded objects from
the network thread (both the channel() accessor and list of
transceivers).

This change adds UpdateNegotiatedChannels to provide a snapshot of
active channels to the network thread before performing operations such
as SetRemoteDescription or RollbackTransports. The lookup map,
negotiated_channels_, is guarded by the network thread and populated
temporarily during these calls.

Abseil's cleanup utility ensures the map is cleared after the transport
operation completes, maintaining proper state lifecycle.

Bug: webrtc:470709349, webrtc:42222849
Change-Id: I7d1ab7c4f5d8cc81feb91c9b4ebeeaf993a6107d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/435761
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46535}