refactor(material/menu): explicitly set aria-expanded to true/false (#19722)

In the context of a submenu trigger it is useful for a screen reader user to
know whether a submenu is open or closed. By specifying the
`aria-expanded` attribute as either true or false it provides an
additional "collapsed" announcement to the user informing them of the
state of the menu trigger when closed.

(cherry picked from commit 6fa02f2)

Author: andy9775

src/material/legacy-menu/menu.spec.ts

@@ -879,7 +879,7 @@ describe('MatMenu', () => {
     fixture.detectChanges();
     const triggerEl = fixture.componentInstance.triggerEl.nativeElement;
 
-    expect(triggerEl.hasAttribute('aria-expanded')).toBe(false);
+    expect(triggerEl.getAttribute('aria-expanded')).toBe('false');
 
     fixture.componentInstance.trigger.openMenu();
     fixture.detectChanges();
@@ -891,7 +891,7 @@ describe('MatMenu', () => {
     fixture.detectChanges();
     tick(500);
 
-    expect(triggerEl.hasAttribute('aria-expanded')).toBe(false);
+    expect(triggerEl.getAttribute('aria-expanded')).toBe('false');
   }));
 
   it('should throw if assigning a menu that contains the trigger', fakeAsync(() => {

src/material/menu/menu-trigger.ts

@@ -78,7 +78,7 @@ export const MENU_PANEL_TOP_PADDING = 8;
 @Directive({
   host: {
     '[attr.aria-haspopup]': 'menu ? "menu" : null',
-    '[attr.aria-expanded]': 'menuOpen || null',
+    '[attr.aria-expanded]': 'menuOpen',
     '[attr.aria-controls]': 'menuOpen ? menu.panelId : null',
     '(click)': '_handleClick($event)',
     '(mousedown)': '_handleMousedown($event)',

src/material/menu/menu.spec.ts

@@ -882,7 +882,7 @@ describe('MDC-based MatMenu', () => {
     fixture.detectChanges();
     const triggerEl = fixture.componentInstance.triggerEl.nativeElement;
 
-    expect(triggerEl.hasAttribute('aria-expanded')).toBe(false);
+    expect(triggerEl.getAttribute('aria-expanded')).toBe('false');
 
     fixture.componentInstance.trigger.openMenu();
     fixture.detectChanges();
@@ -894,7 +894,7 @@ describe('MDC-based MatMenu', () => {
     fixture.detectChanges();
     tick(500);
 
-    expect(triggerEl.hasAttribute('aria-expanded')).toBe(false);
+    expect(triggerEl.getAttribute('aria-expanded')).toBe('false');
   }));
 
   it('should throw if assigning a menu that contains the trigger', fakeAsync(() => {