[SPEAK-ON-MUTE] Add SpeakOnMuteDetected interface.

To support speak-on-mute notification:
  1. add the speak-on-mute detection signal triggered by CRAS.
  2. add method to enable/disable to the speak-on-mute detection.
  3. add method to check the current state of speak-on-mute detection.

This feature is under the flag ash::features::IsSpeakOnMuteEnabled()

The add of the signal on CRAS side has landed on crrev.com/c/3990487

Doc: go/cras-somd-dd
Test: chromeos_unittests
Bug: b/263439990
Change-Id: I8f57e200af7fe5382d66ff93507acbfd60015525
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4141620
Reviewed-by: Yu-Hsuan Hsu <yuhsuan@chromium.org>
Reviewed-by: Charles Zhao <charleszhao@chromium.org>
Commit-Queue: CJ Huang <chenjih@google.com>
Cr-Commit-Position: refs/heads/main@{#1093194}