restructured the add to app samples (#698)

add_to_app/books/flutter_module_books/pigeon/schema.dart

@@ -0,0 +1,32 @@
+// Copyright 2020 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:pigeon/pigeon.dart';
+
+class Book {
+  String title;
+  String subtitle;
+  String author;
+  String description;
+  String publishDate;
+  int pageCount;
+  // Thumbnail thumbnail;
+}
+
+// TODO(gaaclarke): add this back when the https://github.com/flutter/flutter/issues/58896
+// crash is resolved.
+// class Thumbnail {
+//   String url;
+// }
+
+@FlutterApi()
+abstract class FlutterBookApi {
+  void displayBookDetails(Book book);
+}
+
+@HostApi()
+abstract class HostBookApi {
+  void cancel();
+  void finishEditingBook(Book book);
+}