Reduce the amount of C++ code in user space for RN-Tester

Summary:
This diff reduces the amount of C++ code in user space by:
- Moving all the C++ implementation of the .defaults package inside a `react_newarchdefaults` shared library
- Exposing only the entrypoint logic inside the RN-Tester's `OnLoad.cpp` file.

Changelog:
[Android] [Changed] - Reduce the amount of C++ code in user space for New Architecture

Reviewed By: cipolleschi

Differential Revision: D39381820

fbshipit-source-id: 9c4b5596b67b5a7ee58824319c80e325348ed06c

ReactAndroid/src/main/java/com/facebook/react/defaults/DefaultComponentsRegistry.kt

@@ -15,9 +15,8 @@ import com.facebook.react.fabric.ComponentFactory
  * A utility class that provides users a ComponentRegistry they can customize with a C++
  * implementation of its native methods.
  *
- * Please note that you need to provide a native implementation for the method initHybrid for this
- * class, making sure the Java Descriptor is:
- * Lcom/facebook/react/defaults/DefaultComponentsRegistry;
+ * This class works together with the [DefaultNativeEntryPoint] and it's C++ implementation is
+ * hosted inside the React Native framwork
  */
 @DoNotStrip
 class DefaultComponentsRegistry

ReactAndroid/src/main/java/com/facebook/react/defaults/DefaultNativeEntryPoint.kt

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+package com.facebook.react.defaults
+
+import com.facebook.jni.HybridData
+import com.facebook.proguard.annotations.DoNotStrip
+import com.facebook.soloader.SoLoader
+
+/**
+ * A utility class that serves as an entry point for users to register all the custom Fabric
+ * Components and Turbo Native Modules.
+ *
+ * This class needs to be invoked as `DefaultNativeEntryPoint.load("...")` by passing the name of
+ * the dynamic library to load.
+ *
+ * This class works together with the [DefaultNativeEntryPoint] and it's C++ implementation is
+ * hosted inside the React Native framework
+ */
+@DoNotStrip
+class DefaultNativeEntryPoint @DoNotStrip private constructor() {
+
+  @DoNotStrip private val hybridData: HybridData = initHybrid()
+
+  @DoNotStrip private external fun initHybrid(): HybridData
+
+  companion object {
+    @JvmStatic
+    fun load(dynamicLibraryName: String) {
+      SoLoader.loadLibrary("react_newarchdefaults")
+      SoLoader.loadLibrary(dynamicLibraryName)
+      DefaultNativeEntryPoint()
+    }
+  }
+}

ReactAndroid/src/main/java/com/facebook/react/defaults/DefaultTurboModuleManagerDelegate.kt

@@ -17,9 +17,8 @@ import com.facebook.react.bridge.ReactApplicationContext
  * A utility class that allows you to simplify the setup of a
  * [ReactPackageTurboModuleManagerDelegate] for new apps in Open Source.
  *
- * Please note that you need to provide a native implementation for the method initHybrid for this
- * class, making sure the Java Descriptor is:
- * Lcom/facebook/react/defaults/DefaultTurboModuleManagerDelegate;
+ * This class works together with the [DefaultNativeEntryPoint] and it's C++ implementation is
+ * hosted inside the React Native framwork
  */
 class DefaultTurboModuleManagerDelegate
 private constructor(context: ReactApplicationContext, packages: List<ReactPackage>) :