[M148] Fix TOCTOU race in RtpVideoSender::OnVideoLayersAllocationUpdated

RtpVideoSender::OnVideoLayersAllocationUpdated runs on the encoder thread,
checks if it's active, and then posts a task to the transport queue to
update module status.

If SetSending(false) is called on the transport queue before the posted
task runs, the task would previously re-register modules in PacketRouter
even though active_ was false. This lead to dangling pointers in
PacketRouter when the RtpVideoSender was destroyed.

This CL adds an IsActive() check inside the posted task to ensure we don't
re-register modules if the sender has been deactivated.

(cherry picked from commit 77d265670fa7d7f358c86c54704a6e97c27a6005)

Bug: chromium:500767595
Test: RtpVideoSenderTest.PostTaskRaceDoesNotLeadToDanglingPointer
Change-Id: I18827c34bece7f5ebc41d159d9e19e356afbc404
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/462880
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Original-Commit-Position: refs/heads/main@{#47384}
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/465961
Cr-Commit-Position: refs/branch-heads/7778@{#5}
Cr-Branched-From: ca896b7ffef011bbf6957c99d413c5aac602c99f-refs/heads/main@{#47319}