Media service restart is now using BNotification. Fixes 8171.
Media Prefs: rework the BNotification code

* Factor out the notification sending in a single method
* Tweak the "progress" values so the progress bar goes from 0 to 100% in
order, and only once, during the restart
* Remove one notification that was needlessly sent twice
* Some other small cleanups

Final fix for #8171.
Remove now unneeded MediaAlert class.