[place_tracker] ChangeNotifierProvider for state management (#424)

place_tracker/lib/main.dart

@@ -1,7 +1,11 @@
 import 'package:flutter/material.dart';
+import 'package:provider/provider.dart';
 
 import 'place_tracker_app.dart';
 
 void main() {
-  runApp(PlaceTrackerApp());
+  runApp(ChangeNotifierProvider(
+    create: (context) => AppState(),
+    child: PlaceTrackerApp(),
+  ));
 }