extensions-app: Fix service activation mix-up

Starting the D-Bus service should not pop-up a window by itself,
it should just register and wait for an explicit call to Activate().

The .desktop file on the other hand should fully activate the
app when using the Exec line.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7423

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3202>
(cherry picked from commit 3504a6a8a9)

subprojects/extensions-app/data/org.gnome.Extensions.desktop.in.in

@@ -4,7 +4,7 @@ Name=Extensions
 # Translators: Do NOT translate or transliterate this text (this is an icon file name)!
 Icon=@app_id@
 Comment=Configure GNOME Shell Extensions
-Exec=@bindir@/@prgname@ --gapplication-service
+Exec=@bindir@/@prgname@
 DBusActivatable=true
 Categories=GNOME;GTK;Utility;
 OnlyShowIn=GNOME;

subprojects/extensions-app/data/org.gnome.Extensions.service.in

@@ -1,3 +1,3 @@
 [D-BUS Service]
 Name=@app_id@
-Exec=@bindir@/@prgname@
+Exec=@bindir@/@prgname@ --gapplication-service