Move more RTC stats production to signaling thread.

This is CL 2 of 2 that moves population of stats objects from the
networking thread over to the signaling thread.

This CL moves the construction of the RTCStatsReport
(ProducePartialResultsOnNetworkThreadImpl and its helpers) from the
network thread to the signaling thread as well as related operations.

Now that much of the stats processing is done based on data snapshots,
we can more consistently separate the collecting of the data from the
act of populating the stats report

Renamed helper methods from suffix (network thread) to suffix (signaling
thread) to reflect this change:
- ProduceCertificateStats_n -> _s
- ProduceIceCandidateAndPairStats_n -> _s
- ProduceTransportStats_n -> _s
- ProduceRTPStreamStats_n -> _s
- ProduceAudioRTPStreamStats_n -> _s
- ProduceVideoRTPStreamStats_n -> _s

Bug: webrtc:42222804
Change-Id: Ia1f7733bb31fa50c6ca1dc88739ed3068b9d67d2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/445140
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46816}