Add unloadApp method to ReactDelegate (#46424)

Summary:
Pull Request resolved: #46424

It's not isomorphic for `ReactDelegate` to have a `loadApp` method to initialize a surface, but only allow unloading of a surface by calling `onHostDestroy`, which also destroys the ReactHost. This change adds an `unloadApp` method to `ReactDelegate` so it can be used in, e.g., `ReactFragment` to stop a surface without tearing down the host.

## Changelog

[Android][Added] ReactDelegate `unloadApp` methods for unmounting surfaces without destroying ReactHost

Reviewed By: mdvacca

Differential Revision: D62448543

fbshipit-source-id: e99b880f48e857935ee856c37b881bcc8bd22ad0

packages/react-native/ReactAndroid/api/ReactAndroid.api

@@ -179,6 +179,7 @@ public class com/facebook/react/ReactDelegate {
 	public fun onWindowFocusChanged (Z)V
 	public fun reload ()V
 	public fun shouldShowDevMenuOrReload (ILandroid/view/KeyEvent;)Z
+	public fun unloadApp ()V
 }
 
 public class com/facebook/react/ReactFragment : androidx/fragment/app/Fragment, com/facebook/react/modules/core/PermissionAwareActivity {

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

@@ -143,17 +143,10 @@ public void onHostPause() {
   }
 
   public void onHostDestroy() {
+    unloadApp();
     if (ReactFeatureFlags.enableBridgelessArchitecture) {
-      if (mReactSurface != null) {
-        mReactSurface.stop();
-        mReactSurface = null;
-      }
       mReactHost.onHostDestroy(mActivity);
     } else {
-      if (mReactRootView != null) {
-        mReactRootView.unmountReactApplication();
-        mReactRootView = null;
-      }
       if (getReactNativeHost().hasInstance()) {
         getReactNativeHost().getReactInstanceManager().onHostDestroy(mActivity);
       }
@@ -281,10 +274,16 @@ public void reload() {
     devSupportManager.handleReloadJS();
   }
 
+  /** Start the React surface with the app key supplied in the {@link ReactDelegate} constructor. */
   public void loadApp() {
     loadApp(mMainComponentName);
   }
 
+  /**
+   * Start the React surface for the given app key.
+   *
+   * @param appKey The ID of the app to load into the surface.
+   */
   public void loadApp(String appKey) {
     // With Bridgeless enabled, create and start the surface
     if (ReactFeatureFlags.enableBridgelessArchitecture) {
@@ -305,6 +304,21 @@ public void loadApp(String appKey) {
     }
   }
 
+  /** Stop the React surface started with {@link ReactDelegate#loadApp()}. */
+  public void unloadApp() {
+    if (ReactFeatureFlags.enableBridgelessArchitecture) {
+      if (mReactSurface != null) {
+        mReactSurface.stop();
+        mReactSurface = null;
+      }
+    } else {
+      if (mReactRootView != null) {
+        mReactRootView.unmountReactApplication();
+        mReactRootView = null;
+      }
+    }
+  }
+
   @Nullable
   public ReactRootView getReactRootView() {
     if (ReactFeatureFlags.enableBridgelessArchitecture) {