Refactor RTCStatsCollector to have a CollectionContext

Encapsulates state variables used during a stats gathering session into
a new internal `CollectionContext` struct. Previously, these variables
were stored as member variables of `RTCStatsCollector`, despite
implicitly having a shorter lifecycle, associated with the stats
gathering operation (only being valid for the duration of an
asynchronous getStats call).

This change makes further improvements to remove blocking calls simpler.

Bug: webrtc:42222804
Change-Id: I98b2a0f408138d5a65afcee0da8a37eea0cc37bb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/443380
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46732}