This change adds multichannel support to the AEC3 suppressor.
Processing of mono capture is bit-exact to the previous code.
Bug: webrtc:10913
Change-Id: I89affe3e066021bc34e4b525edf44dd3bea68365
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/158882
Commit-Queue: Gustaf Ullberg <gustaf@webrtc.org>
Reviewed-by: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29692}
