Autofill URL and TOKEN in debug mode. (#123)

* Autofill URL and TOKEN in debug mode.

* dart format.

* Set ENV priority higher than prefs.

example/README.md

@@ -1,3 +1,14 @@
 # LiveKit Flutter Example
 
 This app implements a video room using LiveKit's Flutter SDK. Designed to run for iOS, Android, and web.
+
+## Quickstart
+
+Run example:
+
+```bash
+flutter pub get
+# Due to the inconvenience of typing on mobile devices, 
+# you can autofill URL and TOKEN for first run in debug mode.
+flutter run --dart-define=URL=ws://${LIVEKIT_SERVER_IP}:7880 --dart-define=TOKEN=${YOUR_TOKEN}
+```