Lazily creating the FLEXWindow when it's actually needed.

This allows users of `FLEXManager` to use the `-registerGlobalEntryWithName:objectFutureBlock:` API in a `+load` method of their app. Right now that creates the `UIWindow` object when `UIKit` is not ready. With this change, the window will be created the first time you call `-showExplorer`, which would also help a bit with launch times in apps that have FLEX because creating that object would be delayed from app launch to the first time it's used.