Migrate ReactNativeHost from Java to Kotlin

Summary:
Mechanical migration of the deprecated `ReactNativeHost` class from Java to Kotlin. All methods retain their exact JVM signatures to preserve API compatibility.

Key migration decisions:
- Class remains `abstract` (implicitly open)
- All non-final, non-abstract methods marked `open` to match Java virtual-by-default
- `getApplication()` remains `final` (no `open`)
- Methods kept as explicit `fun` (not Kotlin properties) to preserve exact JVM getter names like `getJSMainModuleName()`, `getJSBundleFile()`, `getUIManagerProvider()`
- `Nullable` return types converted to Kotlin nullable types (`?`)
- `synchronized` methods use `Synchronized` annotation
- `Nullsafe` annotation removed (redundant in Kotlin)
- Static initializer converted to companion object `init` block

Changelog:
[Android][Changed] - Migrate `ReactNativeHost` from Java to Kotlin (no behavioral changes)

Reviewed By: javache

Differential Revision: D105013283

Author: cortinico

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/ReactDelegate.kt

@@ -249,7 +249,7 @@ public open class ReactDelegate {
             ((ReactNativeNewArchitectureFeatureFlags.enableBridgelessArchitecture() &&
                 reactHost?.devSupportManager != null) ||
                 (reactNativeHost?.hasInstance() == true &&
-                    reactNativeHost?.useDeveloperSupport == true))
+                    reactNativeHost?.getUseDeveloperSupport() == true))
     ) {
       event.startTracking()
       return true
@@ -270,7 +270,8 @@ public open class ReactDelegate {
         }
       } else {
         if (
-            reactNativeHost?.hasInstance() == true && reactNativeHost?.useDeveloperSupport == true
+            reactNativeHost?.hasInstance() == true &&
+                reactNativeHost?.getUseDeveloperSupport() == true
         ) {
           reactNativeHost?.reactInstanceManager?.showDevOptionsDialog()
           return true