@Override
  public void setAlpha(int alpha) {
    if (alpha != mAlpha) {
      mAlpha = alpha;
      invalidateSelf();
    }
  }
Sets the drawable's alpha value.

@param alpha The alpha value to set, between 0 and 255.
drawee/src/main/java/com/facebook/drawee/drawable/RoundedColorDrawable.java