code: @SuppressWarnings("unchecked")
      @UiThread
      private void expandViews(@NonNull ExpandableWrapper<P, C> parentWrapper, int flatParentPosition) {
          PVH viewHolder;
          for (RecyclerView recyclerView : mAttachedRecyclerViewPool) {
              viewHolder = (PVH) recyclerView.findViewHolderForAdapterPosition(flatParentPosition);
              if (viewHolder != null && !viewHolder.isExpanded()) {
                  viewHolder.setExpanded(true);
                  viewHolder.onExpansionToggled(false);
              }
          }
  
          updateExpandedParent(parentWrapper, flatParentPosition, false);
      }
docs: Calls through to the ParentViewHolder to expand views for each
  RecyclerView the specified parent is a child of.
  <p>
  These calls to the ParentViewHolder are made so that animations can be
  triggered at the ViewHolder level.
  
  @param flatParentPosition The index of the parent to expand