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This CL notifies the hotwording extension of a microphone state change
by adding a hotwordPrivate.onMicrophoneStateChanged event which triggers
an update of the hotwording state.

It also moves the media device observation from HotwordServiceFactory to
HotwordService.

BUG=461787
TBR=estade@chromium.org

Review URL: https://codereview.chromium.org/1047973003

Cr-Commit-Position: refs/heads/master@{#324024}