[web] Updates to push notif service worker for unused-promise Flow lint

Summary:
Two changes here:

1. Passing skipWaiting directly in self.addEventListener('install', skipWaiting). It looks like the types work for this.
2. Modifying the NotificationEvent.waitUntil call to wait until openWindow completes. I think the unused-promise Lint was helpful here in catching a case we should await, but would appreciate close review.

Depends on D10164

Test Plan: Flow

Reviewers: marcin, michal

Reviewed By: michal

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10165