* Add an explicit `-[FlutterViewController init]` implementation

`-[FlutterViewController init]` currently works because it inherits
the `-[UIViewController init]` convenience initializer that invokes
the `-[UIViewController initWithNibName:bundle:]` designated
initializer that `FlutterViewController` overrides.

However, this doesn't seem to be explicitly documented, so it's a bit
confusing (or at least non-obvious), and it seems potentially
brittle.  Add an explicit implementation of `-[FlutterViewController
init]` instead.