Phone auth can lose verificationId with no way of getting it back

[TikiTDO]

Issue

When using firebase phone authentication, the app can lose a verificationId in between calls to signInWithPhoneNumber and _confirmVerificationCode.

This is touched upon in this comment.

Since signInWithPhoneNumber already passes the verificationId to JS-land, it would be nice to have an alternate validation function such as _confirmVerificationCodeWithId so that this lib would not have to worry about managing that part of the life cycle.

I can submit a PR for Android part, and I could half-ass it for iOS, but I figured it would be better to ask.

Environment

1. Application Target Platform: Android (Though likely affects both)

2. Development Operating System: Linux

3. Build Tools: Android Oreo

4. React Native version: 0.48.3

5. RNFirebase Version: 3.0.0-alpha.3

6. Firebase Module: auth

[chrisbianca]

@TikiTDO See: #119 (comment) for information about what we're proposing in terms of a more bespoke API.

We're aware of the fact that the verificationId can be lost as part of the lifecycle of the apps in Android - though this is already handled in iOS.

You can already make use of the verificationId yourself using a PhoneCredential directly:

const credential = firebase.auth.PhoneAuthProvider.credential(verificationID, authCode);
return firebase.auth().signInWithCredential(credential)

Closing this as we'll pick up as part of #119