function updateActiveOption() {
    var selectedOption = elements.scroller.querySelector('.selected');
    if (selectedOption) {
      ctrl.activeOption = selectedOption.id;
    } else {
      ctrl.activeOption = null;
    }
  }
Update the activeOption based on the selected item in the listbox.
The activeOption is used in the template to set the aria-activedescendant attribute, which
enables screen readers to properly handle visual focus within the listbox and announce the
item's place in the list. I.e. "List item 3 of 50". Anytime that `ctrl.index` changes, this
function needs to be called to update the activeOption.