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/credentials_manager.dart

@@ -0,0 +1,34 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_breez_liquid_example/services/keychain.dart';
+
+class CredentialsManager {
+  static const String accountMnemonic = "account_mnemonic";
+
+  final KeyChain keyChain;
+
+  CredentialsManager({required this.keyChain});
+
+  Future storeMnemonic({required String mnemonic}) async {
+    try {
+      await _storeMnemonic(mnemonic);
+      debugPrint("Stored credentials successfully");
+    } catch (err) {
+      throw Exception(err.toString());
+    }
+  }
+
+  Future<String> restoreMnemonic() async {
+    try {
+      String mnemonicStr = await keyChain.read(accountMnemonic) ?? "";
+      debugPrint("Restored credentials successfully");
+      return mnemonicStr;
+    } catch (err) {
+      throw Exception(err.toString());
+    }
+  }
+
+  // Helper methods
+  Future<void> _storeMnemonic(String mnemonic) async {
+    await keyChain.write(accountMnemonic, mnemonic);
+  }
+}