Add an explicit -[FlutterViewController init] implementation (flutter/engine#5924)

* 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.

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

@@ -84,6 +84,10 @@
   return [self initWithProject:nil nibName:nil bundle:nil];
 }
 
+- (instancetype)init {
+  return [self initWithProject:nil nibName:nil bundle:nil];
+}
+
 #pragma mark - Common view controller initialization tasks
 
 - (void)performCommonViewControllerInitialization {