fix(a11y): don't handle disallowed modifier keys in typeahead mode

Recently we switched to skip handling keyboard events with modifiers, unless the consumer opted into them explicitly. The changes weren't applied when in typeahead mode.

Fixes angular#14274.

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

@@ -664,6 +664,18 @@ describe('Key managers', () => {
         expect(keyManager.activeItem).toBe(itemList.items[1]);
       }));
 
+      it('should not move focus if a modifier, that is not allowed, is pressed', fakeAsync(() => {
+        const tEvent = createKeyboardEvent('keydown', 84, undefined, 't');
+        Object.defineProperty(tEvent, 'ctrlKey', {get: () => true});
+
+        expect(keyManager.activeItem).toBeFalsy();
+
+        keyManager.onKeydown(tEvent); // types "t"
+        tick(debounceInterval);
+
+        expect(keyManager.activeItem).toBeFalsy();
+      }));
+
       it('should focus the first item that starts with sequence of letters', fakeAsync(() => {
         keyManager.onKeydown(createKeyboardEvent('keydown', 84, undefined, 't')); // types "t"
         keyManager.onKeydown(createKeyboardEvent('keydown', 72, undefined, 'h')); // types "h"

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

@@ -246,10 +246,12 @@ export class ListKeyManager<T extends ListKeyManagerOption> {
       default:
         // Attempt to use the `event.key` which also maps it to the user's keyboard language,
         // otherwise fall back to resolving alphanumeric characters via the keyCode.
-        if (event.key && event.key.length === 1) {
-          this._letterKeyStream.next(event.key.toLocaleUpperCase());
-        } else if ((keyCode >= A && keyCode <= Z) || (keyCode >= ZERO && keyCode <= NINE)) {
-          this._letterKeyStream.next(String.fromCharCode(keyCode));
+        if (isModifierAllowed) {
+          if (event.key && event.key.length === 1) {
+            this._letterKeyStream.next(event.key.toLocaleUpperCase());
+          } else if ((keyCode >= A && keyCode <= Z) || (keyCode >= ZERO && keyCode <= NINE)) {
+            this._letterKeyStream.next(String.fromCharCode(keyCode));
+          }
         }
 
         // Note that we return here, in order to avoid preventing