* Added notificationListner sample code

* Fixed spacing

* fixed spacing

* Moved the variables into the Build method

* fixed typos

* Update packages/flutter/lib/src/widgets/notification_listener.dart
Co-authored-by: John Ryan <ryjohn@google.com>

* Update packages/flutter/lib/src/widgets/notification_listener.dart
Co-authored-by: John Ryan <ryjohn@google.com>

* Update packages/flutter/lib/src/widgets/notification_listener.dart
Co-authored-by: John Ryan <ryjohn@google.com>

* Update packages/flutter/lib/src/widgets/notification_listener.dart
Co-authored-by: John Ryan <ryjohn@google.com>

* Update packages/flutter/lib/src/widgets/notification_listener.dart
Co-authored-by: John Ryan <ryjohn@google.com>

* Update packages/flutter/lib/src/widgets/notification_listener.dart
Co-authored-by: John Ryan <ryjohn@google.com>

* Update packages/flutter/lib/src/widgets/notification_listener.dart
Co-authored-by: John Ryan <ryjohn@google.com>