[firebase_auth] fix inconsistency between iOS and Android on FirebaseUserMetadata

[long1eu]

Description

On iOS creationTimestamp and lastSignInTimestamp were return in seconds and on Android in milliseconds. Now both platforms return those values in milliseconds (edit: as a DateTime).

Related Issues

flutter/flutter#27986

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[collinjackson]

This is a breaking change to the behavior and we're doing some other breaking changes to the library right now (see #1911) so I think it's a good time to update these method names and have them return a DateTime, which is more idiomatic for Dart. I've added these changes to your PR.