extensions-app: Start as service when D-Bus activated

When the app is properly launched, D-Bus activation will be followed
by a call to org.freedesktop.Application.Activate() to open the window.

But it's also possible for D-Bus activation to happen in other
circumstances, for example during gdbus command line completion.

We don't want to pop up a window then, so pass the --gapplication-service
flag to properly separate D-Bus activation from activating the app.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2076>
(cherry picked from commit e8d5564e)