Revert RCTRootView's backgroundColor to white (facebook#23358)

Summary:
Fixes facebook#23314 , the change came from facebook#20945 , its purpose is to fix orientation change issue in HelloWord template, but I think it's just a trick, to make rootView's backgroundColor the same as window backgroundColor, the orientation issue seems related to [UIManager setAvailableSize:forRootView:](https://github.com/facebook/react-native/blob/d2fc19f4aa94888b7c3d3f4a5fb621bf96a1aff9/React/Modules/RCTUIManager.m#L343) async layout things.

[iOS] [Fixed] - Revert RCTRootView's backgroundColor to white.
Pull Request resolved: facebook#23358

Differential Revision: D14030666

Pulled By: cpojer

fbshipit-source-id: 7c9a45f03b87c3be0f2b7c64a3c837c6ae14af3e

Author: zhongwuzw

template/ios/HelloWorld/AppDelegate.m

@@ -20,7 +20,7 @@ - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(
                                                    moduleName:@"HelloWorld"
                                             initialProperties:nil];
 
-  rootView.backgroundColor = [UIColor blackColor];
+  rootView.backgroundColor = [[UIColor alloc] initWithRed:1.0f green:1.0f blue:1.0f alpha:1];
 
   self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
   UIViewController *rootViewController = [UIViewController new];