fix(modal): capture and return focus

Focus is captured on a focusable element in the modal when it opened,
and focus is returned to the previously-focused element when it is
closed.

This change contributes to fixing VPAT-735.

projects/angular/src/modal/modal.html

@@ -4,7 +4,7 @@
   ~ The full license information can be found in LICENSE in the root directory of this project.
   -->
 
-<div cdkTrapFocus class="modal" *ngIf="_open">
+<div cdkTrapFocus [cdkTrapFocusAutoCapture]="true" class="modal" *ngIf="_open">
   <!--fixme: revisit when ngClass works with exit animation-->
   <div
     [@fadeDown]="skipAnimation"