Extract collection of BWE stats from DefaultVideoQualityAnalyzer to separate class to prepare for migration on new GetStats API and simplify quality analyzer. Bug: webrtc:11381 Change-Id: I0e7e2d7e40b467d7a42633a72a7ffc49ebcb0237 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/169444 Commit-Queue: Artem Titov <titovartem@webrtc.org> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org> Cr-Commit-Position: refs/heads/master@{#30650}