[systemd] Report xembedsniproxy as up only after we're actually started

[systemd] Report xembedsniproxy as up only after we're actually started and have registered to the X11 window.

This way we completely reduce the race with X11 applications that don't correctly monitor system tray availability.

If we fail to claim the selection we exit which systemd should also notice.

Reported via https://bugzilla.redhat.com/show_bug.cgi?id=2038750

FWIW, legacy boot is also racey, just slower - but we may as well fix things now we have the tools