add packages for flutter secure storage

lib/screen_loading.dart

@@ -5,16 +5,19 @@ class ScreenLoading extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return Column(children: const <Widget>[
-      SizedBox(
-        width: 60,
-        height: 60,
-        child: CircularProgressIndicator(),
-      ),
-      Padding(
-        padding: EdgeInsets.only(top: 16),
-        child: Text('Awaiting result...'),
-      )
-    ]);
+    return Column(
+        mainAxisAlignment: MainAxisAlignment.center,
+        crossAxisAlignment: CrossAxisAlignment.center,
+        children: const <Widget>[
+          SizedBox(
+            width: 60,
+            height: 60,
+            child: CircularProgressIndicator(),
+          ),
+          Padding(
+            padding: EdgeInsets.only(top: 16),
+            child: Text('Awaiting result...'),
+          )
+        ]);
   }
 }