android: Exit process after activity shutdown

Hack to help with #1216 a bit

Unfortunately this causes the activity to reopen sometimes, but maybe this can serve as a stopgap for now

pandatool/src/deploy-stub/android_main.cxx

@@ -332,4 +332,6 @@ void android_main(struct android_app *app) {
 
   // Detach the thread before exiting.
   activity->vm->DetachCurrentThread();
+
+  _exit(0);
 }