Changing default values of multi channel processing to be true

This CL changes the default values of multi channel processing to be true. The CL will be followed by another CL that removes these parameters once their use in Chromium has been removed.

The reason for their removal is that their initial purpose, which was allowing a fallback to single-channel processing until the multi-channel processing functionality was verified is no longer needed.

The CL also includes an update of the unittests where the tests was including an expectation on a code-independent test threshold value. That change is needed as the threshold took effect as part of this code-change.
A follow-up CL will be made to refactor that test.


Bug: chromium:464314991
Change-Id: Ie4141cd5b13c6b748f1ff1552b526c15155ac279
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/429343
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Commit-Queue: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46856}