Update example app on Flutter plugin (#220)

* Update example app on Flutter plugin

* Expose `empty_wallet_cache` through Dart bindings (#224)

packages/flutter/example/lib/services/keychain.dart

@@ -0,0 +1,21 @@
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+
+class KeyChain {
+  final FlutterSecureStorage _storage = const FlutterSecureStorage();
+
+  Future<String?> read(String key) {
+    return _storage.read(key: key);
+  }
+
+  Future write(String key, String value) {
+    return _storage.write(key: key, value: value);
+  }
+
+  Future delete(String key) {
+    return _storage.delete(key: key);
+  }
+
+  Future clear() {
+    return _storage.deleteAll();
+  }
+}