fix(autocomplete): suggestions can be highlighted incorrectly (#11529)

using ng-class within md-virtual-repeat doesn't seem to be working
it was evaluating the following to true: -1 === 0, -1 === 1, etc.

Fixes #10573.

Author: Splaktar

src/components/autocomplete/autocomplete-theme.scss

@@ -51,7 +51,7 @@ md-autocomplete.md-THEME_NAME-theme {
   li {
     color: '{{foreground-1}}';
     &:hover,
-    &.selected {
+    &#selected_option {
       background: '{{background-500-0.18}}';
     }
   }

src/components/autocomplete/js/autocompleteDirective.js

@@ -368,7 +368,6 @@ function MdAutocomplete ($$mdSvgRegistry) {
                 id="ul-{{$mdAutocompleteCtrl.id}}"\
                 role="listbox">\
               <li md-virtual-repeat="item in $mdAutocompleteCtrl.matches"\
-                  ng-class="{ selected: $index === $mdAutocompleteCtrl.index }"\
                   ng-attr-id="{{$index === $mdAutocompleteCtrl.index ? \'selected_option\' : undefined}}"\
                   ng-click="$mdAutocompleteCtrl.select($index)"\
                   role="option"\