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

test/unit-tests/components/views/settings/tabs/user/SessionManagerTab-test.tsx

@@ -631,9 +631,10 @@ describe("<SessionManagerTab />", () => {
             // click verify button from current session section
             fireEvent.click(getByTestId(`verification-status-button-${alicesMobileDevice.device_id}`));
 
-            const { onFinished: modalOnFinished } = modalSpy.mock.calls[0][1] as any;
-            // simulate modal completing process
-            await modalOnFinished();
+            // close the modal
+            const { close: closeModal } = modalSpy.mock.results[0].value;
+            closeModal();
+            await flushPromises();
 
             // cancelled in case it was a failure exit from modal
             expect(mockVerificationRequest.cancel).toHaveBeenCalled();