refactor(select): use aria-activedescendant to manage focus

* Refactors the select to use `aria-activedescendant` to announce the highlighted item to screen readers. Previously we would this through focus, however using focus prevents us from being able to do things like #3211.
* Fixes a hack that was used to get a hold of the panel element using `querySelector`. Now it properly uses a `ViewChild` query, however this meant some tests had to be updated.

Relates to #3211.
Fixes #6690.

Author: crisbeto

src/lib/select/select.html

@@ -34,16 +34,19 @@
   (detach)="close()">
 
   <div
+    #panel
     class="mat-select-panel {{ 'mat-' + color }}"
     [ngClass]="panelClass"
     [@transformPanel]="multiple ? 'showing-multiple' : 'showing'"
     (@transformPanel.done)="_onPanelDone()"
-    (keydown)="_handlePanelKeydown($event)"
     [style.transformOrigin]="_transformOrigin"
     [class.mat-select-panel-done-animating]="_panelDoneAnimating"
     [style.font-size.px]="_triggerFontSize">
 
-    <div class="mat-select-content" [@fadeInContent]="'showing'" (@fadeInContent.done)="_onFadeInDone()">
+    <div
+      class="mat-select-content"
+      [@fadeInContent]="'showing'"
+      (@fadeInContent.done)="_onFadeInDone()">
       <ng-content></ng-content>
     </div>
   </div>