Make RtpVideoSender::OnBitrateUpdated lock-free

SetFecAllowed() is called by the encoder, and OnBitrateUpdated() reads
the value on the transport queue. This was the last reason for
OnBitrateUpdated() to take mutex_, which blocked the worker thread
whenever a bitrate update coincided with packetization of an encoded
frame.

Post the value to the transport queue instead and guard it with
transport_checker_. FecControllerOverride is public API, so this works
for calls from any thread.

Bug: webrtc:42223727
Change-Id: Iad4757cfd3092f39fda1086fcb942c3d56c5b8bf
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/506163
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48789}