GitLab

Move playout channel selection and mono mixing to StreamMixer to
simplify the logic.

Bug: internal b/118685514
Change-Id: I6c2d871d23958d615c4bf828b5b6013929e3bea0
Reviewed-on: https://chromium-review.googlesource.com/c/1347207Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610991}