Don't drop key frame of re-enabled simulcast layer

RtpVideoSender activates RTP modules in a task on the transport queue,
so the encoder could produce the key frame of a re-enabled layer while
the module wasn't sending yet. The key frame was then dropped, and the
layer was unusable until the receiver requested a new key frame.

Enable media synchronously in OnVideoLayersAllocationUpdated(). The rest
of the activation, including the pacer registration, still runs on the
transport queue, before the frame's packets reach the pacer.

Fixed: webrtc:566494723
Change-Id: I5e4260a99f53c86fcdc798a6279d416ca6d50ac4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/505681
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48754}