* When the user isn't requesting a custom notification, it will
be a BMediaRoster job to do it.
* Reintroduce BMediaRoster::SyncToServices, this time based on local
message passing rather than a global semaphore.
* SyncToServices is used in launch_media_server to make the process
more launch_daemon safe and faster in the average case.
* It was an error to add notifications in the media_server.
* Fixes #12717.
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