fix(snack-bar): announcing same message twice to screen readers (#13298)

Currently we have `role="alert"` on the snack bar which will cause screen readers to announce the message automatically. On top of it, we also use the `LiveAnnouncer` to announce the same message, if the consumer hasn't set one. These changes clear the `announcementMessage` if it's the same as the main message.

src/lib/snack-bar/snack-bar.spec.ts

@@ -186,18 +186,16 @@ describe('MatSnackBar', () => {
   }));
 
 
-  it('should default to the passed message for the announcement message', fakeAsync(() => {
+  it('should clear the announcement message if it is the same as main message', fakeAsync(() => {
     spyOn(liveAnnouncer, 'announce');
 
-    snackBar.open(simpleMessage);
+    snackBar.open(simpleMessage, undefined, {announcementMessage: simpleMessage});
     viewContainerFixture.detectChanges();
 
     expect(overlayContainerElement.childElementCount)
       .toBe(1, 'Expected the overlay with the default announcement message to be added');
 
-    // Expect the live announcer to have been called with the display message and some
-    // string for the politeness. We do not want to test for the default politeness here.
-    expect(liveAnnouncer.announce).toHaveBeenCalledWith(simpleMessage, jasmine.any(String));
+    expect(liveAnnouncer.announce).not.toHaveBeenCalled();
   }));
 
   it('should be able to specify a custom announcement message', fakeAsync(() => {

src/lib/snack-bar/snack-bar.ts

@@ -114,8 +114,10 @@ export class MatSnackBar implements OnDestroy {
     // override the data to pass in our own message and action.
     _config.data = {message, action};
 
-    if (!_config.announcementMessage) {
-      _config.announcementMessage = message;
+    // Since the snack bar has `role="alert"`, we don't
+    // want to announce the same message twice.
+    if (_config.announcementMessage === message) {
+      _config.announcementMessage = undefined;
     }
 
     return this.openFromComponent(SimpleSnackBar, _config);