Clarify sample rate support in media::AudioProcessor

The WebRTC audio processing module has had some minor API clarifications, this CL lifts these clarification to the media::AudioProcessor APM and implementation, and updates the unit tests to cover all input sample rates observed in telemetry.

Bug: chromium:1332484, chromium:1334991
Change-Id: I8ee2348d8e17eed5ca464d7e3ce2480e18ea711b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3913087


Commit-Queue: Sam Zackrisson <saza@chromium.org>
Reviewed-by: Ale Bzk <alessiob@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1054198}