code: protected AnimatorSet addListenersToSet(AnimatorSet set) {
          if (set != null) {
              set.addListener(this);
              if (mListeners != null) {
                  for (Animator.AnimatorListener listener : mListeners) {
                      set.addListener(listener);
                  }
              }
          }
          return set;
      }
docs: Adds the current listeners to the {@link android.animation.AnimatorSet}
  before animation starts
  
  @param set The set to add listeners to
  @return The set with listeners added