feat(firestore, android): allow FirestoreSerializer native use

[mikehardy]

Description

I have a brownfield use case where I do some firestore things in native code while doing most in Javascript.
For best code re-use in the app I sometimes want to send objects to the native code for further decoration before persisting to Firestore

Being able to re-use the existing serializer for that would be fantastic, and it's just two "public" markings away...

Release Summary

feat(firestore, android): allow FirestoreSerializer native re-use in brownfield apps

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • Yes
• My change supports the following platforms;
  • Android
  • iOS
• My change includes tests;
  • e2e tests added or updated in packages/\*\*/e2e
  • jest tests added or updated in packages/\*\*/__tests__
• I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • Yes
  • No

Test Plan

Using it already in my project via patch-package

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[Salakar]

LGTM!