[firebase_core] Minor cleanups

[swift-kim]

No description provided.

[swift-kim]

I just noticed that there's a common analysis_options.yaml for Firebase Desktop packages. Our policy when implementing third-party plugins is to respect "their" analysis options and copy their analysis_options.yaml to the package root (example). However, as we are eventually going to have multiple FlutterFire plugin implementations in this repo, we could also create a common analysis options file in the repo root just like the plus plugins' case.

So there are three options:

1. No change ("their" code should follow "our" analysis options when being copied to this repo).
2. Let each FlutterFire package in this repo have its own analysis_options.yaml and all_lint_rules.yaml files.
3. Create a common analysis_options_firebase.yaml in the repo root and import it from each FlutterFire package.

I'll go with the first option if you guys don't have a specific idea.

cc @wiertel