Revert "Removed the configuration options for multi_channel processing"

This reverts commit db6adf2e9f9a213f9045e1c388713fe727245727.

Reason for revert: Breaks webrtc=>chrome roll https://crrev.com/c/7672366

Original change's description:
> Removed the configuration options for multi_channel processing
>
> This CL removes the explicit configuration options for whether
> the audio processing should be performed in a multi- or
> single-channel manner.
> This change means that the choice of whether to do multi- or
> single-channel audio processing will be made based on the
> content of the audio sent to APM for processing, and the
> audio output requested by APM.
>
> Bug: chromium:464314991
> Change-Id: I55a53f2565c81a7d7ad2e378f944ffcf07c71f4f
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/453000
> Commit-Queue: Per Åhgren <peah@webrtc.org>
> Reviewed-by: Sam Zackrisson <saza@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#47175}

Bug: chromium:464314991
Change-Id: If2888c379fe3a5bfdac6c12225a12705a1377c30
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/457640
Commit-Queue: Evan Shrubsole <eshr@webrtc.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47182}