fix(material/autocomplete): add missing aria-label for autocomplete panel

Author: annieyw

src/material/autocomplete/autocomplete.html

@@ -1,5 +1,5 @@
 <ng-template>
-  <div class="mat-autocomplete-panel" role="listbox" [id]="id" [ngClass]="_classList" #panel>
+  <div class="mat-autocomplete-panel" role="listbox" aria-label="Search suggestions" [id]="id" [ngClass]="_classList" #panel>
     <ng-content></ng-content>
   </div>
 </ng-template>