Android: Make FlutterNativeAdLoadedListener more generally usable

In `FlutterNativeAdLoadedListener`, make the inner weak reference
be generic on the `OnNativeAdLoadedListener` interface instead of
the `FlutterNativeAd` concrete class. This allows using the same
listener class with any other type that conforms to
`OnNativeAdLoadedListener`.

Update `FlutterNativeAd` to conform to `OnNativeAdLoadedListener`
to allow delegating in the same existing fashion.

Author: msbit

packages/google_mobile_ads/android/src/main/java/io/flutter/plugins/googlemobileads/FlutterAdListener.java

@@ -97,16 +97,16 @@ public void onAdLoaded() {
 /** {@link OnNativeAdLoadedListener} for native ads. */
 class FlutterNativeAdLoadedListener implements OnNativeAdLoadedListener {
 
-  private final WeakReference<FlutterNativeAd> nativeAdWeakReference;
+  private final WeakReference<OnNativeAdLoadedListener> listenerWeakReference;
 
-  FlutterNativeAdLoadedListener(FlutterNativeAd flutterNativeAd) {
-    nativeAdWeakReference = new WeakReference<>(flutterNativeAd);
+  FlutterNativeAdLoadedListener(OnNativeAdLoadedListener listener) {
+    listenerWeakReference = new WeakReference<>(listener);
   }
 
   @Override
   public void onNativeAdLoaded(@NonNull NativeAd nativeAd) {
-    if (nativeAdWeakReference.get() != null) {
-      nativeAdWeakReference.get().onNativeAdLoaded(nativeAd);
+    if (listenerWeakReference.get() != null) {
+      listenerWeakReference.get().onNativeAdLoaded(nativeAd);
     }
   }
 }

packages/google_mobile_ads/android/src/main/java/io/flutter/plugins/googlemobileads/FlutterNativeAd.java

@@ -32,7 +32,7 @@
 import java.util.Map;
 
 /** A wrapper for {@link NativeAd}. */
-class FlutterNativeAd extends FlutterAd {
+class FlutterNativeAd extends FlutterAd implements OnNativeAdLoadedListener {
   private static final String TAG = "FlutterNativeAd";
 
   @NonNull private final AdInstanceManager manager;
@@ -248,7 +248,8 @@ public PlatformView getPlatformView() {
     return null;
   }
 
-  void onNativeAdLoaded(@NonNull NativeAd nativeAd) {
+  @Override
+  public void onNativeAdLoaded(@NonNull NativeAd nativeAd) {
     if (nativeTemplateStyle != null) {
       templateView = nativeTemplateStyle.asTemplateView(context);
       templateView.setNativeAd(nativeAd);