Capture channel pointers on signaling thread to fix data race

The stats collector previously accessed channel getters directly from
transceivers during the collection process. This created a data race if
a transceiver's channel was cleared on the signaling thread while the
collection was still ongoing/queued on the worker thread.

This change introduces a callback-based mechanism for retrieving media
channel statistics and RTP parameters. By capturing these as
absl::AnyInvocable objects on the signaling thread at the start of the
stats collection process, the collector can safely execute them later on
the worker thread.

Bug: webrtc:481443652
Change-Id: Ia8ee334df710bc8655e2617e079bf200ab04173e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/461260
Reviewed-by: Evan Shrubsole <eshr@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47317}