Convert RTCStatsCollector to unique_ptr ownership

In the previous CL, we removed dependency on reference counting from
within the RTCStatsCollector class. Instead using thread specific safety
flag. That allows us to transition RTCStatsCollector from being a
reference-counted object to unique ownership.

This CL removes the RefCountInterface dependency and updates
PeerConnection to manage the collector's lifetime via std::unique_ptr.

Along the way update the tests to utilize test::RunLoop and update
RTCStatsObtainer accordingly to handle asynchronous stats delivery.

Bug: None
Change-Id: I0d8d7d6bbfbe60228e71f07374e8e5a56fec7955
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/436040
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46537}