Updates SubtractorOutputAnalyzer and AecState::SaturationDetector to multi-channel. Bug: webrtc:10913 Change-Id: I39edafdc5d5a4db5cc853cf116d60af0f506b3bf Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/154342 Commit-Queue: Sam Zackrisson <saza@webrtc.org> Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org> Reviewed-by: Per Åhgren <peah@webrtc.org> Cr-Commit-Position: refs/heads/master@{#29355}