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);
        }
    }
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.
android/modules/module_filepicker/src/main/java/com/nononsenseapps/filepicker/AbstractFilePickerFragment.java