Modal: remove support for onFinished callback (#29852)

* Fix up type for `finished` result of Modal

The `finished` promise can be called with an empty array, for example if the
dialog is closed by a background click. This was not correctly represented in
the typing. Fix that, and add some documentation while we're at it.

* Type fixes to onFinished callbacks from Modal

These can all be called with zero arguments, despite what the type annotations
may say, so mark them accordingly.

* Remove uses of Modal `onFinished` property

... because it is confusing.

Instead, use the `finished` promise returned by `createDialog`.

* Modal: remove support for now-unused `onFinished` prop

* StopGapWidgetDriver: use `await` instead of promise chaining

* Fix up unit tests

src/async-components/views/dialogs/security/NewRecoveryMethodDialog.tsx

@@ -49,15 +49,8 @@ export default function NewRecoveryMethodDialog({ onFinished }: NewRecoveryMetho
         if (isKeyBackupEnabled) {
             onFinished();
         } else {
-            Modal.createDialog(
-                RestoreKeyBackupDialog,
-                {
-                    onFinished,
-                },
-                undefined,
-                false,
-                true,
-            );
+            const { finished } = Modal.createDialog(RestoreKeyBackupDialog, {}, undefined, false, true);
+            finished.then(onFinished);
         }
     }