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/routes/connect/restore_page.dart

@@ -0,0 +1,104 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+
+class RestorePage extends StatefulWidget {
+  final Future Function(String mnemonic) onRestore;
+
+  const RestorePage({super.key, required this.onRestore});
+
+  @override
+  State<RestorePage> createState() => _RestorePageState();
+}
+
+class _RestorePageState extends State<RestorePage> {
+  final _formKey = GlobalKey<FormState>();
+  List<TextEditingController> textFieldControllers =
+      List<TextEditingController>.generate(12, (_) => TextEditingController());
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      home: Scaffold(
+        appBar: AppBar(
+          backgroundColor: Colors.white,
+          actions: [
+            IconButton(
+              onPressed: () async {
+                final clipboardData = await Clipboard.getData('text/plain');
+                final clipboardMnemonics = clipboardData?.text?.split(" ");
+                if (clipboardMnemonics?.length == 12) {
+                  for (var i = 0; i < clipboardMnemonics!.length; i++) {
+                    textFieldControllers.elementAt(i).text = clipboardMnemonics.elementAt(i);
+                  }
+                }
+              },
+              icon: const Icon(Icons.paste, color: Colors.blue),
+            ),
+          ],
+        ),
+        body: Padding(
+          padding: const EdgeInsets.symmetric(horizontal: 16.0, vertical: 8.0),
+          child: Form(
+            key: _formKey,
+            child: GridView.builder(
+              gridDelegate: SliverGridDelegateWithMaxCrossAxisExtent(
+                maxCrossAxisExtent: MediaQuery.of(context).size.width / 2,
+                childAspectRatio: 2,
+                crossAxisSpacing: 8,
+                mainAxisSpacing: 8,
+              ),
+              itemCount: 12,
+              itemBuilder: (BuildContext context, int index) {
+                return TextFormField(
+                  decoration: InputDecoration(labelText: "${index + 1}"),
+                  inputFormatters: [FilteringTextInputFormatter.deny(RegExp(r"\s\b|\b\s"))],
+                  validator: (String? value) {
+                    if (value == null || value.isEmpty) {
+                      return 'Please enter value';
+                    }
+                    return null;
+                  },
+                  controller: textFieldControllers[index],
+                );
+              },
+            ),
+          ),
+        ),
+        bottomNavigationBar: Padding(
+          padding: EdgeInsets.only(
+            bottom: MediaQuery.of(context).viewInsets.bottom + 40.0,
+          ),
+          child: Column(
+            mainAxisSize: MainAxisSize.min,
+            children: [
+              ElevatedButton(
+                style: ElevatedButton.styleFrom(
+                  backgroundColor: Colors.blue,
+                  elevation: 0.0,
+                  shape: RoundedRectangleBorder(
+                    borderRadius: BorderRadius.circular(8.0),
+                  ),
+                ),
+                onPressed: () async {
+                  if (_formKey.currentState?.validate() ?? false) {
+                    final mnemonic = textFieldControllers
+                        .map((controller) => controller.text.toLowerCase().trim())
+                        .toList()
+                        .join(" ");
+                    widget.onRestore(mnemonic);
+                  }
+                },
+                child: Text(
+                  "RESTORE",
+                  textAlign: TextAlign.center,
+                  style: Theme.of(context).primaryTextTheme.titleMedium,
+                  maxLines: 1,
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}