Fix orientation change issue in HelloWord template (facebook#20945)

Summary:
This commit is related to issue facebook#16060
Tested through the iOS simulator. Solution came from https://moduscreate.com/blog/changing-the-react-native-rootview-background-color-ios-and-android/
Pull Request resolved: facebook#20945

Differential Revision: D9661761

Pulled By: hramos

fbshipit-source-id: ca8dd3fd09a0b0e89e1598a8114800808efa7d21

local-cli/templates/HelloWorld/ios/HelloWorld/AppDelegate.m

@@ -22,7 +22,7 @@ - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(
                                                       moduleName:@"HelloWorld"
                                                initialProperties:nil
                                                    launchOptions:launchOptions];
-  rootView.backgroundColor = [[UIColor alloc] initWithRed:1.0f green:1.0f blue:1.0f alpha:1];
+  rootView.backgroundColor = [UIColor blackColor];
 
   self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
   UIViewController *rootViewController = [UIViewController new];