Make Linux apps non-unique by default. (#74959)

This stops them attempting to own a D-Bus name, which may not be possible if the
app is sandboxed (e.g. in a Snap). It's also probably what most developers would
expect is the correct behaviour by default.

packages/flutter_tools/templates/app/linux.tmpl/my_application.cc.tmpl

@@ -100,5 +100,6 @@ static void my_application_init(MyApplication* self) {}
 MyApplication* my_application_new() {
   return MY_APPLICATION(g_object_new(my_application_get_type(),
                                      "application-id", APPLICATION_ID,
+                                     "flags", G_APPLICATION_NON_UNIQUE,
                                      nullptr));
 }