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If the user updates the pref (e.g., via chrome://settings) then they
presumably already know about always on hotwording so there's no reason to
annoy them with a notification. This CL adds a pref registrar observer
to the always on pref and when it's changed, marks the notification as seen
and then removes the observer.

This will only work for new users since once they have the always on pref,
this observer will get hit on switching between profiles, but presumably
once they have the always on pref, it means they enabled it or disabled it
at some point.

BUG=443805

Review URL: https://codereview.chromium.org/815223003

Cr-Commit-Position: refs/heads/master@{#309516}