* FirebaseMessaging: `NSUserDefaults` replaced by `GULUserDefaults`
* FirebaseMessaging: `GoogleUtilities/UserDefaults` added to dependencies. `GoogleUtilities` dependencies version updated to `~> 5.3`
* FIRMessagingPubSub: imports order
* Prevent Messaging and IID singleton usage during tests.
* Remove other uses of static Messaging instance.
* Removed usages of messagingForTests.
* Remove unused imports, unify testing code.
* Use local import path instead of framework import.