fix(material/datepicker): fix Voiceover losing focus on PageDown

Fixes an issue where Voiceover loses focus when pressing PageDown/PageUp
in the calendar to go to the next month/year (issue angular#24330). Adding a
20ms timeout seems to fix this.

Note that this will not fully fix the issue until angular#24397 is merged.

Address angular#24330.

src/material/datepicker/calendar-body.ts

@@ -20,7 +20,7 @@ import {
   OnDestroy,
   AfterViewChecked,
 } from '@angular/core';
-import {take} from 'rxjs/operators';
+import {delay, take} from 'rxjs/operators';
 
 /** Extra CSS classes that can be associated with a calendar cell. */
 export type MatCalendarCellCssClasses = string | string[] | Set<string> | {[key: string]: any};
@@ -31,6 +31,8 @@ export type MatCalendarCellClassFunction<D> = (
   view: 'month' | 'year' | 'multi-year',
 ) => MatCalendarCellCssClasses;
 
+export const FOCUS_ACTIVE_CELL_DELAY = 20;
+
 /**
  * An internal class that represents the data corresponding to a single calendar cell.
  * @docs-private
@@ -216,10 +218,14 @@ export class MatCalendarBody implements OnChanges, OnDestroy, AfterViewChecked {
     return cellNumber == this.activeCell;
   }
 
-  /** Focuses the active cell after the microtask queue is empty. */
+  /**
+   * Focuses the active cell after the microtask queue is empty.
+   *
+   * Adds a 20ms delay to fix Voiceover losing focus when pressing PageUp/PageDown (issue #24330).
+   */
   _focusActiveCell(movePreview = true) {
     this._ngZone.runOutsideAngular(() => {
-      this._ngZone.onStable.pipe(take(1)).subscribe(() => {
+      this._ngZone.onStable.pipe(take(1), delay(FOCUS_ACTIVE_CELL_DELAY)).subscribe(() => {
         const activeCell: HTMLElement | null = this._elementRef.nativeElement.querySelector(
           '.mat-calendar-body-active',
         );