main: Activate notification proxy on start

main: Activate notification proxy on start

At least for the time being, this looks like the easiest option to
launch the service:

 - we could add a systemd unit, but then we'd need to update the
   RequiredComponents in the fallback session definition as well,
   making it necessary for gnome-shell, gnome-shell-extensions and
   gnome-session to be updated to 3.36.1 in lockstep

 - autostart is problematic as it would make gnome-shell conflict
   with other notification daemons; also autostart is most useful
   with automatic shutdown, which would require tracking signal
   subscriber to determine when the service is unused

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/547