[in_app_purchase_android] Fix mockMethodCallHandler lint (flutter#3674)

Author: bparrishMines

packages/in_app_purchase/in_app_purchase_android/test/billing_client_wrappers/billing_client_manager_test.dart

@@ -26,8 +26,9 @@ void main() {
   const String onBillingServiceDisconnectedCallback =
       'BillingClientStateListener#onBillingServiceDisconnected()';
 
-  setUpAll(() =>
-      channel.setMockMethodCallHandler(stubPlatform.fakeMethodCallHandler));
+  setUpAll(() => _ambiguate(TestDefaultBinaryMessengerBinding.instance)!
+      .defaultBinaryMessenger
+      .setMockMethodCallHandler(channel, stubPlatform.fakeMethodCallHandler));
 
   setUp(() {
     WidgetsFlutterBinding.ensureInitialized();
@@ -105,3 +106,9 @@ void main() {
     });
   });
 }
+
+/// This allows a value of type T or T? to be treated as a value of type T?.
+///
+/// We use this so that APIs that have become non-nullable can still be used
+/// with `!` and `?` on the stable branch.
+T? _ambiguate<T>(T? value) => value;