code: public void onClickCheckBox(@NonNull CheckableViewHolder viewHolder) {
          if (mCheckedItems.contains(viewHolder.file)) {
              viewHolder.checkbox.setChecked(false);
              mCheckedItems.remove(viewHolder.file);
              mCheckedVisibleViewHolders.remove(viewHolder);
          } else {
              if (!allowMultiple) {
                  clearSelections();
              }
              viewHolder.checkbox.setChecked(true);
              mCheckedItems.add(viewHolder.file);
              mCheckedVisibleViewHolders.add(viewHolder);
          }
      }
docs: Called when a selectable item's checkbox is pressed. This should toggle its selected state.
  Note that if only a single item can be selected, then other potentially selected views on
  screen must be de-selected. The text box for new filename is also cleared.
  
  @param viewHolder for the item containing the checkbox.