Add swingmusic-mobile submodule to replace Android app

- Updated .gitmodules to include mobile app submodule
- Added swingmusic_mobile directory with Flutter app
- Mobile app will now be built in unified release workflow

swingmusic_mobile/linux/runner/main.cc

@@ -0,0 +1,6 @@
+#include "my_application.h"
+
+int main(int argc, char** argv) {
+  g_autoptr(MyApplication) app = my_application_new();
+  return g_application_run(G_APPLICATION(app), argc, argv);
+}