[system-ui] fix RCTRootViewBackgroundColor setup from plugin (expo#25946

)

# Why

expo-system-ui config plugin will setup `RCTRootViewBackgroundColor`
from app.json's `ios.backgroundColor` to info.plist. from expo#22773, we
break this and expo-system-ui will always use system default color as
the background color:
https://github.com/expo/expo/blob/e281adb30e978c382220686ffb0cb1b665f8d3c5/packages/expo-system-ui/ios/ExpoSystemUI/ExpoSystemUIModule.swift#L44-L60

# How

if the color is not persisted yet, we should respect the color from
`RCTRootViewBackgroundColor`

packages/expo-system-ui/CHANGELOG.md

@@ -8,6 +8,8 @@
 
 ### 🐛 Bug fixes
 
+- Fixed custom `backgroundColor` is broken on iOS. ([#25946](https://github.com/expo/expo/pull/25946) by [@kudo](https://github.com/kudo))
+
 ### 💡 Others
 
 ## 2.9.0 — 2023-12-12

packages/expo-system-ui/ios/ExpoSystemUI/ExpoSystemUIModule.swift

@@ -8,14 +8,14 @@ public class ExpoSystemUIModule: Module {
     Name("ExpoSystemUI")
 
     OnCreate {
-      // TODO: Maybe read from the app manifest instead of from Info.plist.
-      // Set / reset the initial color on reload and app start.
       let color = UserDefaults.standard.integer(forKey: Self.colorKey)
-
       if color > 0 {
         Self.setBackgroundColorAsync(color: color)
       } else {
-        Self.setBackgroundColorAsync(color: nil)
+        // TODO: Maybe read from the app manifest instead of from Info.plist.
+        // Set / reset the initial color on reload and app start.
+        let colorFromPlugin = Bundle.main.object(forInfoDictionaryKey: "RCTRootViewBackgroundColor") as? Int
+        Self.setBackgroundColorAsync(color: colorFromPlugin)
       }
     }