fix(a11y): activeItem out of date if active index is removed from ListKeyManager (#14407)

Fixes the `activeItem` on the `ListKeyManager` not matching the item at the `activeItemIndex`, if the `activeItem` is removed from the list.

Fixes #14345.

src/cdk/a11y/key-manager/list-key-manager.spec.ts

@@ -93,6 +93,17 @@ describe('Key managers', () => {
       expect(keyManager.activeItem!.getLabel()).toBe('one');
     });
 
+    it('should keep the active item in sync if the active item is removed', () => {
+      expect(keyManager.activeItemIndex).toBe(0);
+      expect(keyManager.activeItem!.getLabel()).toBe('one');
+
+      itemList.items.shift();
+      itemList.notifyOnChanges();
+
+      expect(keyManager.activeItemIndex).toBe(0);
+      expect(keyManager.activeItem!.getLabel()).toBe('two');
+    });
+
     it('should start off the activeItem as null', () => {
       expect(new ListKeyManager([]).activeItem).toBeNull();
     });

src/cdk/a11y/key-manager/list-key-manager.ts

@@ -67,8 +67,8 @@ export class ListKeyManager<T extends ListKeyManagerOption> {
           const itemArray = newItems.toArray();
           const newIndex = itemArray.indexOf(this._activeItem);
 
-          if (newIndex > -1 && newIndex !== this._activeItemIndex) {
-            this._activeItemIndex = newIndex;
+          if (newIndex !== this._activeItemIndex) {
+            this.updateActiveItem(newIndex > -1 ? newIndex : this._activeItemIndex);
           }
         }
       });