change back to use animatedicons in gallery

Samples/UIWidgetsSamples_2019_4/Assets/UIWidgetsGallery/demo/material/backdrop_demo.cs

                     {
                         new IconButton(
                             onPressed: this._toggleBackdropPanelVisibility,
-                            icon: new Icon(
-                                icon: Icons.close
+                            icon: new AnimatedIcon(
+                                icon: AnimatedIcons.close_menu,
+                                progress: this._controller.view
                             )
                         )
                     }

Samples/UIWidgetsSamples_2019_4/Assets/UIWidgetsGallery/demo/material/search_demo.cs

                 appBar: new AppBar(
                     leading: new IconButton(
                         tooltip: "Navigation menu",
-                        icon: new Icon(
-                            icon: Icons.menu,
-                            color: Colors.white
+                        icon: new AnimatedIcon(
+                            icon: AnimatedIcons.arrow_menu,
+                            color: Colors.white,
+                            progress: this._delegate.transitionAnimation
                         ),
                         onPressed: () => { this._scaffoldKey.currentState.openDrawer(); }
                     ),
         public override Widget buildLeading(BuildContext context) {
             return new IconButton(
                 tooltip: "Back",
-                icon: new Icon(
-                    icon: Icons.menu
+                icon: new AnimatedIcon(
+                    icon: AnimatedIcons.arrow_menu,
+                    progress: this.transitionAnimation
                 ),
                 onPressed: () => { this.close(context, null); }
             );

Samples/UIWidgetsSamples_2019_4/Assets/UIWidgetsGallery/gallery/backdrop.cs

                             trailing: new IconButton(
                                 onPressed: this._toggleFrontLayer,
                                 tooltip: "Toggle options page",
-                                icon: new Icon(
-                                    icon: Icons.close
+                                icon: new AnimatedIcon(
+                                    icon: AnimatedIcons.close_menu,
+                                    progress: this._controller
                                 )
                             )
                         ),