Make the argument speech_probability non-optional in InputVolumeController::Process() and MonoInputVolumeController::Process(). Additional clean-up: Remove the flag enabled in the config. Add unit tests for MonoInputVolumeController. Bug: webrtc:7494 Change-Id: Ie28af77dc628bf71d09ce1ff033d39031f77a21e Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/283700 Reviewed-by: Alessio Bazzica <alessiob@webrtc.org> Commit-Queue: Hanna Silen <silen@webrtc.org> Cr-Commit-Position: refs/heads/main@{#38710}