Allow packageName and applicationId to be different

## What feature would you like to see?

Not sure if this is the correct repository to report this, but it's Android-related, so I'll go ahead.

There wasn't much difference between Android's `packageName` and `applicationId` a long time ago.
Now `packageName`, aka `namespace`, can be configured independently of the `applicationId`.

[Configure the app module | Android Developers](https://developer.android.com/studio/build/configure-app-module#change_the_application_id_for_testing)

But, Firebase still treats `packageName` and `applicationId` as the same.

[Add Firebase to your Android project | Firebase Documentation](https://firebase.google.com/docs/android/setup#create-firebase-project)

## How would you use it?

I love to have the ability to change the package name/namespace without any troubles with Firebase SDK.