code: ViewPager.OnPageChangeListener setInternalPageChangeListener(ViewPager.OnPageChangeListener listener) {
          ViewPager.OnPageChangeListener oldListener = mInternalPageChangeListener;
          mInternalPageChangeListener = listener;
          return oldListener;
      }
docs: Set a separate OnPageChangeListener for internal use by the support library.
  
  @param listener Listener to set
  @return The old listener that was set, if any.