[TabAlert] Reduce TabAlertController audible sensitivity

The TabAlertController was observing live audio status and forwarding
it to observers which can be more sensitive to small pauses in audio.
This CL changes the alert controller to subscribe to the
RecentlyAudibleHelper instead which has a built in 2 second delay
before notifying subscribers that audio has stopped; this makes it so
that the behavior of the TabAlertController matches that of existing
alerts in the TabRendererData.

Fixed: 427213113
Change-Id: I9faf9d584ac504e5bae5afc6795ef304d50e0428
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6787413
Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Reviewed-by: Darryl James <dljames@chromium.org>
Commit-Queue: Steven Luong <stluong@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1492313}