[native] Silence Flow errors from react-native-firebase
[native] Silence Flow errors from react-native-firebase

Summary:
I spent quite a bit of time fixing errors in this package yesterday (D1737). After enabling `types-first`, we ended up seeing more errors as a result of typing `getFirebase()`.

This library is increasingly outdated and we're going to reimplement notifications logic as part of the upcoming E2E encryption work. Instead of wasting more time typing it, I'm going to just silence the Flow errors coming from it.

Test Plan: Flow

Reviewers: atul, palys-swm

Reviewed By: atul

Subscribers: KatPo, Adrian

Differential Revision: https://phabricator.ashoat.com/D1742

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