Autocomplete should use role=combobox

Typically editable combo boxes are used for autocomplete behavior, and authors SHOULD set aria-autocomplete attribute on the textfield.
https://www.w3.org/TR/wai-aria/roles#combobox

Closes #7941

Author: tirana

src/components/autocomplete/js/autocompleteDirective.js

@@ -174,8 +174,7 @@ function MdAutocomplete () {
       return '\
         <md-autocomplete-wrap\
             layout="row"\
-            ng-class="{ \'md-whiteframe-z1\': !floatingLabel, \'md-menu-showing\': !$mdAutocompleteCtrl.hidden }"\
-            role="listbox">\
+            ng-class="{ \'md-whiteframe-z1\': !floatingLabel, \'md-menu-showing\': !$mdAutocompleteCtrl.hidden }">\
           ' + getInputElement() + '\
           <md-progress-linear\
               class="' + (attr.mdFloatingLabel ? 'md-inline' : '') + '"\
@@ -251,6 +250,7 @@ function MdAutocomplete () {
                   ' + (attr.mdSelectOnFocus != null ? 'md-select-on-focus=""' : '') + '\
                   aria-label="{{floatingLabel}}"\
                   aria-autocomplete="list"\
+                  role="combobox"\
                   aria-haspopup="true"\
                   aria-activedescendant=""\
                   aria-expanded="{{!$mdAutocompleteCtrl.hidden}}"/>\
@@ -276,6 +276,7 @@ function MdAutocomplete () {
                 ' + (attr.mdSelectOnFocus != null ? 'md-select-on-focus=""' : '') + '\
                 aria-label="{{placeholder}}"\
                 aria-autocomplete="list"\
+                role="combobox"\
                 aria-haspopup="true"\
                 aria-activedescendant=""\
                 aria-expanded="{{!$mdAutocompleteCtrl.hidden}}"/>\