Set defaultUseMaterial3 = false due to Flutter issue 107190

To avoid having this issue visible as default in Theme Playground
we default useMaterial3 until it is fixed or behaves better
flutter/flutter#107190

example/lib/shared/const/store.dart

@@ -203,7 +203,7 @@ class Store {
   // Key used to read and save the useMaterial3 value.
   static const String keyUseMaterial3 = 'useMaterial3';
   // Default value for the useMaterial3, also to reset settings.
-  static const bool defaultUseMaterial3 = true;
+  static const bool defaultUseMaterial3 = false;
 
   // Key used to read and save the useKeyColors value.
   static const String keyUseKeyColors = 'useKeyColors';