xdg-autostart-generator: use Type=exec

We check that the binary exists before writing the service file, but
let's also not consider the service started until the fork has happened.
This is still relatively new stuff, so we're can change the implementation
details like this.

src/xdg-autostart-generator/xdg-autostart-service.c

@@ -604,7 +604,7 @@ int xdg_autostart_service_generate_unit(
 
         fprintf(f,
                 "\n[Service]\n"
-                "Type=simple\n"
+                "Type=exec\n"
                 "ExecStart=:%s\n"
                 "Restart=no\n"
                 "TimeoutSec=5s\n"