code: @UiThread
      protected void parentExpandedFromViewHolder(int flatParentPosition) {
          ExpandableWrapper<P, C> parentWrapper = mFlatItemList.get(flatParentPosition);
          updateExpandedParent(parentWrapper, flatParentPosition, true);
      }
docs: Called when a ParentViewHolder has triggered an expansion for it's parent
  
  @param flatParentPosition the position of the parent that is calling to be expanded