code: public void onSaveInstanceState(Bundle outState) {
      for (Screen screen : backStack) {
        screen.save(outState);
        screen.onSave(outState);
      }
    }
docs: Notifies all screens to save instance state in input Bundle.
  
  Call this method from {@link Activity#onSaveInstanceState(Bundle) onSaveInstanceState} in the Activity associated
  with this Navigator
  
  @param outState  Bundle in which to store screen state information