@Override public void setBackgroundDrawable(Drawable d) {
    super.setBackgroundDrawable(d);
    if (!mDrawFullUnderlineSet) {
      mDrawFullUnderline = d == null;
    }
  }
@return The current tab indicator color as an 0xRRGGBB value.
sample/src/main/java/android/support/v4/view/PagerTabStripV22.java