Audit v9 public APIs to eliminate naming inconsistencies for functions, types, etc.

[Feiyang1]

Currently we have some inconsistent naming in v9 API. For example, the service types(the type of instances returned from get functions) are named differently in different packages.

FirebaseFirestore and FirebaseDatabase are prefixed with Firebase, while others are not (e.g. Auth, Functions). And StorageService is different from all others.

We should review the entire API, find inconsistencies, come up with rules and remediate them.

[hsubox76]

It seems Performance also uses FirebasePerformance.

[Feiyang1]

Fixed by #5135
#5136 and
#5137