code: @NonNull
      public IconicsAnimationProcessor addListener(@NonNull IconicsAnimationListener listener) {
          if (mListeners == null) {
              mListeners = new ArrayList<>();
              mAnimator.addListener(mProxyListener);
          }
          mListeners.add(listener);
          return this;
      }
docs: Adds a listener to the set of listeners that are sent events through the life of an
  processor, such as start, repeat, and end.
  
  @param listener the listener to be added to the current set of listeners for this processor.