Add aria-modal where appropriate (#11684)

* fix(dialog): add `aria-modal="true"`

The `aria-modal` attribute was added in ARIA 1.1

* fix(bottom-sheet): add `aria-modal="true"`

The `aria-modal` attribute was added in ARIA 1.1

Author: jelbourn

src/lib/bottom-sheet/bottom-sheet-container.ts

@@ -52,6 +52,7 @@ import {FocusTrap, FocusTrapFactory} from '@angular/cdk/a11y';
     'class': 'mat-bottom-sheet-container',
     'tabindex': '-1',
     'role': 'dialog',
+    'aria-modal': 'true',
     '[attr.aria-label]': 'bottomSheetConfig?.ariaLabel',
     '[@state]': '_animationState',
     '(@state.start)': '_onAnimationStart($event)',

src/lib/dialog/dialog-container.ts

@@ -59,6 +59,7 @@ export function throwMatDialogContentAlreadyAttachedError() {
   host: {
     'class': 'mat-dialog-container',
     'tabindex': '-1',
+    'aria-modal': 'true',
     '[attr.id]': '_id',
     '[attr.role]': '_config.role',
     '[attr.aria-labelledby]': '_config.ariaLabel ? null : _ariaLabelledBy',