@Override
  protected void onSizeChanged(int width, int height, int oldWidth, int oldHeight) {
    setMeasuredDimension(width, height);
    calculateDotPositions();
  }
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