Add Flutter GUI example for VAD with a microphone. (#905)

sherpa-onnx/flutter/example/pubspec.yaml

@@ -32,7 +32,9 @@ dependencies:
     sdk: flutter
 
   path_provider: ^2.1.3
-  path:
+  path: ^1.9.0
+  record: ^5.1.0
+  url_launcher: ^6.2.6
 
   sherpa_onnx:
     # When depending on this package from a real application you should use:
@@ -71,8 +73,8 @@ flutter:
   # To add assets to your application, add an assets section, like this:
   assets:
     - assets/
-    - assets/sr-data/enroll/
-    - assets/sr-data/test/
+  #   - assets/sr-data/enroll/
+  #   - assets/sr-data/test/
   #   - images/a_dot_ham.jpeg
 
   # An image asset can refer to one or more resolution-specific "variants", see