[core] fix splash flickering in dark mode (expo#26015)

# Why

aside from expo#25946 and expo#25971, i also noticed a splash flickering without
expo-system-ui. this one is coming from RCTRootView setup.

# How

we didn't setup backgroundColor [as react-native](https://github.com/facebook/reactnative/blob/43826facfab8eed7d01a801778d1c477e60730a6/packages/react-native/Libraries/AppDelegate/RCTAppDelegate.mm#L154).
in dark mode after splash screen and before App fully loaded, it will show white screen in between. this pr tries to set the RCTRootView's background color as system background color.

- Note that with expo-system-ui, it may override the RCTRootView's
background color from it's OnCreate lifecycle.
- The downside of this pr's approach is that, ReactDelegate subscribers
have no way to change the background color. i think it is okay if we
don't support the use case.

packages/expo-modules-core/CHANGELOG.md

@@ -9,6 +9,7 @@
 ### 🐛 Bug fixes
 
 - [iOS] Fixed `SharedObjectRegistry` crash for accessing internal data structures from multi-threads. ([#25997](https://github.com/expo/expo/pull/25997) by [@kudo](https://github.com/kudo))
+- Fixed splash screen view flickering in dark mode on iOS. ([#26015](https://github.com/expo/expo/pull/26015) by [@kudo](https://github.com/kudo))
 
 ### 💡 Others
 

packages/expo-modules-core/ios/AppDelegates/EXAppDelegateWrapper.mm

@@ -65,10 +65,12 @@ - (UIView *)createRootViewWithBridge:(RCTBridge *)bridge
   enableFabric = self.fabricEnabled;
 #endif
 
-  return [self.reactDelegate createRootViewWithBridge:bridge
-                                         moduleName:moduleName
-                                    initialProperties:initProps
-                                        fabricEnabled:enableFabric];
+  UIView *rootView = [self.reactDelegate createRootViewWithBridge:bridge
+                                                       moduleName:moduleName
+                                                initialProperties:initProps
+                                                    fabricEnabled:enableFabric];
+  rootView.backgroundColor = UIColor.systemBackgroundColor;
+  return rootView;
 }
 
 - (UIViewController *)createRootViewController