Move FirebaseMessaging to `GULUserDefaults`

[ryanwilson]

Currently Messaging uses NSUserDefaults for some persistent storage, but it should move to GULUserDefaults to avoid the crashing bug in iOS 11 due to UI being updated on the wrong thread by iOS.

[morganchen12]

@ryanwilson can we close this issue?

[ryanwilson]

Looks like it's still outstanding and relevant. Should be a very quick fix, though.