refactor: linting de widgets de layout y utilidades

Ajustes de estilo/linting en los helpers centered* de layout,
appIntroPage, roundedButton, secretLoader y utils (refactor pendiente).

lib/layout/centeredMaxWidget.dart

@@ -4,10 +4,10 @@ import 'centeredRow.dart';
 
 class CenteredMaxWidget extends CenteredColumn {
   CenteredMaxWidget({
-    Key key,
-    TextDirection textDirection,
-    TextBaseline textBaseline,
-    List<Widget> children: const <Widget>[],
+    Key? key,
+    TextDirection? textDirection,
+    TextBaseline? textBaseline,
+    List<Widget> children = const <Widget>[],
   }) : super(
     children: <Widget>[new CenteredRow(children: children)],
           key: key,