Add a new audio extension event OnMuteChanged.

This is the part of audio extension redesign efforts. The original audio extension only has one OnDeviceChanged event without any input arguments to notify all sorts of audio events, which is not efficient. The hotrod app has to query getInfo to get all sets of the devices with properties, comparing the old data to decide what change really happens and handles the change. Therefore, in the new design, we will add new events to accurately report what exact audio change occurs. Eventually, we will migrate to use the new events and retire the old OnDeviceChanged event.

See details in design doc:
https://docs.google.com/a/google.com/document/d/1YFFLwX4mcKJyuAsZB13GhDid0MEt0QguKX0AxgvBoko/edit?usp=sharing

BUG=429312
TBR=mnissler

Review URL: https://codereview.chromium.org/1033603006

Cr-Commit-Position: refs/heads/master@{#322907}

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