Allow more than 2 input channels in AudioProcessing.

The number of output channels is constrained to be equal to either 1 or the
number of input channels.

An earlier version of this commit caused a crash on AEC dump.

TBR=aluebs@webrtc.org,pbos@webrtc.org

Review URL: https://codereview.webrtc.org/1248393003 .

Cr-Commit-Position: refs/heads/master@{#9626}
13 files changed