fix: remove exposed internal variable on width (#4)

Author: omariosouto

example/with_flutter_sample/pubspec.lock

@@ -227,7 +227,7 @@ packages:
       path: "../../package"
       relative: true
     source: path
-    version: "2.1.3"
+    version: "2.3.0"
   sky_engine:
     dependency: transitive
     description: flutter

package/lib/responsive/screen_width.dart

@@ -1,12 +1,12 @@
 class ScreenWidth {
-  final double widthValue;
-  const ScreenWidth(this.widthValue);
+  final double _widthValue;
+  const ScreenWidth(this._widthValue);
 
   double get value {
-    return widthValue;
+    return _widthValue;
   }
 
   double percent(double percent) {
-    return widthValue * (percent / 100);
+    return _widthValue * (percent / 100);
   }
 }