fix: ensure aria-activedescendant is correct

This is a fix for a severe error found using the Google Accessibility Developer Tool audit:
ARIA attributes which refer to other elements by ID should refer to elements which exist in the DOM.

src/bootstrap/select.tpl.html

@@ -4,7 +4,6 @@
          aria-expanded="true"
          aria-label="{{ $select.baseTitle }}"
          aria-owns="ui-select-choices-{{ $select.generatedId }}"
-         aria-activedescendant="ui-select-choices-row-{{ $select.generatedId }}-{{ $select.activeIndex }}"
          class="form-control ui-select-search"
          ng-class="{ 'ui-select-search-hidden' : !$select.searchEnabled }"
          placeholder="{{$select.placeholder}}"

src/select2/select.tpl.html

@@ -12,7 +12,6 @@
        aria-expanded="true"
        aria-owns="ui-select-choices-{{ $select.generatedId }}"
        aria-label="{{ $select.baseTitle }}"
-       aria-activedescendant="ui-select-choices-row-{{ $select.generatedId }}-{{ $select.activeIndex }}"
              class="ui-select-search select2-input"
              ng-model="$select.search">
     </div>

src/uiSelectController.js

@@ -740,4 +740,16 @@ uis.controller('uiSelectCtrl',
     ctrl.searchInput.off('keyup keydown tagged blur paste');
     angular.element($window).off('resize', onResize);
   });
+
+  $scope.$watch('$select.activeIndex', function(activeIndex) {
+    if (activeIndex)
+      $element.find('input').attr(
+        'aria-activedescendant',
+        'ui-select-choices-row-' + ctrl.generatedId + '-' + activeIndex);
+  });
+
+  $scope.$watch('$select.open', function(open) {
+    if (!open)
+      $element.find('input').removeAttr('aria-activedescendant');
+  });
 }]);