switch to cdkAriaLive

Author: mmalerba

src/lib/datepicker/calendar-header.html

@@ -2,7 +2,7 @@
   <div class="mat-calendar-controls">
     <button mat-button type="button" class="mat-calendar-period-button"
             (click)="currentPeriodClicked()" [attr.aria-label]="periodButtonLabel"
-            aria-live="polite">
+            cdkAriaLive="polite">
       {{periodButtonText}}
       <div class="mat-calendar-arrow"
            [class.mat-calendar-invert]="calendar.currentView != 'month'"></div>