Construct Call on the signaling thread

Move the construction of the Call object owned by PeerConnection from
the worker thread to the signaling thread.

This change:
* Removes thread checks in Call constructor and moves repeating tasks
  startup and stats registration to EnsureStarted().
* Updates RtpTransportControllerSend to defer pacer configuration
  and BWE callback registration to EnsureStarted().
* Updates WebRtcVoiceEngine to make audio_state_ const and thread safe.
* Updates PeerConnectionFactory to construct Call on signaling thread
  and safely destroy metronomes on worker thread.

Bug: webrtc:42222117
Change-Id: I88771e127c61b280da29bde8ef9d83e3d74ad06b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/479480
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47917}