endSessionDialog: Immediately return from Open()

endSessionDialog: Immediately return from Open()

There's no need to wait for the dialog to finish fading in before we can
return from Open().

The code, as it was before, would break dialog updates: the dialog was
already open, so the signal would never get emitted, so the method would
never return, and so gnome-session would remain blocked waiting for a
return value that will never come.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4040>