firebase-ios-sdk

Add GULUserDefaults to GoogleUtilities. (#1812)

* Add GULUserDefaults to GoogleUtilities.

This is a workaround for NSUserDefaults crashing when setting values
in the background in some situations on iOS 11. This uses the C API
in order to avoid an NSNotification firing (which a class eventually
tries to update the UI from the thread that set the value).

* Add missing copyright information.

* Resolve macOS issues, address feedback.

* Removed unnecessary enum value.