[core] fix splash flickering in dark mode

[Kudo]

Why

aside from #25946 and #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. 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.

Test Plan

as #25971

$ yarn create expo -t blank@sdk-50 sdk50
$ cd sdk 50
$ patch << EOF
--- a/App.js
+++ b/App.js
@@ -13,7 +13,7 @@ export default function App() {
 const styles = StyleSheet.create({
   container: {
     flex: 1,
-    backgroundColor: '#fff',
+    backgroundColor: '#000',
     alignItems: 'center',
     justifyContent: 'center',
   },
--- a/app.json
+++ b/app.json
@@ -5,11 +5,11 @@
     "version": "1.0.0",
     "orientation": "portrait",
     "icon": "./assets/icon.png",
-    "userInterfaceStyle": "light",
+    "userInterfaceStyle": "dark",
     "splash": {
       "image": "./assets/splash.png",
       "resizeMode": "contain",
-      "backgroundColor": "#ffffff"
+      "backgroundColor": "#000000"
     },
     "assetBundlePatterns": [
       "**/*"
EOF
$ npx expo run:ios

# should not keep all screen in black

Checklist

• Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• Conforms with the Documentation Writing Style Guide
• This diff will work correctly for npx expo prebuild & EAS Build (eg: updated a module plugin).