This CL adds proper multi-channel support to the analog AGC. Beyond that, it prepares adding multi-channel support to the digital AGC by removing the tight dependency between the analog and digital AGC codes. Bug: webrtc:10859 Change-Id: I4414ccbc3db5dbb5ae069fdf426cbd038375ca7b Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/159480 Reviewed-by: Sam Zackrisson <saza@webrtc.org> Commit-Queue: Per Åhgren <peah@webrtc.org> Cr-Commit-Position: refs/heads/master@{#29878}