Pass the correct number of channels needed by the AGC2 input volume controller. This change doesn't affect the adaptive digital controller which reads the number of channel from the passed audio buffer instance for each processed frame. Note that the `AdaptiveDigitalGainController::Initialize()` impl was removed in [1], but that CL didn't remove the declaration (done in this CL). [1] https://webrtc-review.googlesource.com/c/src/+/287222/5/modules/audio_processing/agc2/adaptive_digital_gain_controller.cc#105 Bug: webrtc:7494 Change-Id: I07369ab4025a251b25c716cf618e4222fdb60fc8 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/287320 Reviewed-by: Hanna Silen <silen@webrtc.org> Commit-Queue: Alessio Bazzica <alessiob@webrtc.org> Cr-Commit-Position: refs/heads/main@{#38863}