media-keys: Use the shell's ShowOSD() method for OSDs

Now that fallback mode has been dropped and we rely on the shell,
there's no longer any good reason to implement OSDs in g-s-d
(a background service!) while trying to mimick the shell style
as close as possible. Instead, delegate showing the actual UI
to the shell.

https://bugzilla.gnome.org/show_bug.cgi?id=695021