Standardize on GoogleUtilities Swizzler

[paulb777]

Standardize swizzler usage with the GoogleUtilities Swizzler in FirebaseAuth and FirebaseMessaging.

[paulb777]

Contact @tejasd when ready to start working on a PR for this.

[maksymmalyhin]

Classes to be attended:

• FIRAuthAppDelegateProxy
• FIRMessagingRemoteNotificationsProxy