Add a policy for Focus Mode Sounds

Policy currently allows for 3 states: Everything (YTM + Focus Sounds)
enabled, just Focus Sounds (no YTM), or Disabled (Sounds completely
disabled). Updates to the UI to follow.

Bug: b:328121041
Change-Id: Ia673b7ab6a7d94a68eff5dc2d6582c2c545d95b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5547974
Auto-Submit: Sean Kau <skau@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Igor <igorcov@chromium.org>
Commit-Queue: Sean Kau <skau@chromium.org>
Reviewed-by: Richard Chui <richui@chromium.org>
Reviewed-by: Igor Ruvinov <igorruvinov@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1310713}