[messaging] Change import in service worker

[jhuleatt]

The reference docs show a separate getMessaging function specifically for service workers. This PR updates the sample to use that instead of from firebase/messaging.

In my testing, importing getMessaging from firebase/messaging caused errors such as:

TypeError: Cannot read property 'addEventListener' of undefined

and

FirebaseError: Messaging: This browser doesn't support indexedDb.open() (ex. Safari iFrame, Firefox Private Browsing, etc) (messaging/indexed-db-unsupported).

[jhuleatt]

leaving this as draft until firebase/firebase-js-sdk#5314 is resolved

[egilmorez]

Good catch, thanks!