Android: Allow passing a FlutterAdLoader factory

Create a new `GoogleMobileAdsPlugin` constructor which takes an additional
`Supplier<FlutterAdLoader>` argument. This is available solely for testing,
and allows the provision of a mock `FlutterAdLoader` for those `FlutterAd`s
that need it during their construction.

Author: msbit

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

@@ -43,6 +43,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.function.Supplier;
 
 /**
  * Flutter plugin accessing Google Mobile Ads API.
@@ -69,6 +70,7 @@ private static <T> T requireNonNull(T obj) {
   private final Map<String, NativeAdFactory> nativeAdFactories = new HashMap<>();
   @Nullable private MediationNetworkExtrasProvider mediationNetworkExtrasProvider;
   private final FlutterMobileAdsWrapper flutterMobileAds;
+  @Nullable private Supplier<FlutterAdLoader> adLoaderSupplier;
   /**
    * Public constructor for the plugin. Dependency initialization is handled in lifecycle methods
    * below.
@@ -94,6 +96,17 @@ protected GoogleMobileAdsPlugin(@NonNull AppStateNotifier appStateNotifier) {
     this.flutterMobileAds = new FlutterMobileAdsWrapper();
   }
 
+  @VisibleForTesting(otherwise = VisibleForTesting.NONE)
+  protected GoogleMobileAdsPlugin(
+      @Nullable FlutterPluginBinding pluginBinding,
+      @Nullable AdInstanceManager instanceManager,
+      @NonNull FlutterMobileAdsWrapper flutterMobileAds,
+      @NonNull Supplier<FlutterAdLoader> adLoaderSupplier) {
+    this(pluginBinding, instanceManager, flutterMobileAds);
+
+    this.adLoaderSupplier = adLoaderSupplier;
+  }
+
   /**
    * Interface used to display a {@link com.google.android.gms.ads.nativead.NativeAd}.
    *