@Override
  protected void onDraw(Canvas canvas) {
    if (viewPager == null || pageCount == 0) {
      return;
    }
    drawUnselected(canvas);
    drawSelected(canvas);
  }
As this class <b>must</b> act as the {@link ViewPager.OnPageChangeListener} for the ViewPager
(as set by {@link #setViewPager(android.support.v4.view.ViewPager)}).  Applications may set a
listener here to be notified of the ViewPager events.

@param onPageChangeListener
java/com/google/android/apps/authenticator/howitworks/PagingIndicator.java