Fix some issues.

Runtime/material/app_bar.cs

         public override float? maxExtent {
             get {
                 return Mathf.Max(
-                    this.topPadding ?? 0.0f + (this.expandedHeight ?? Constants.kToolbarHeight + this._bottomHeight),
+                    (this.topPadding ?? 0.0f) + (this.expandedHeight ?? Constants.kToolbarHeight + this._bottomHeight),
                     this.minExtent ?? 0.0f);
             }
         }

Samples/UIWidgetsGallery/demo/contacts_demo.cs

                                         fit: StackFit.expand,
                                         children: new List<Widget> {
                                             Image.asset(
-                                                "ali_landscape.png",
+                                                "ali_landscape",
                                                 fit: BoxFit.cover,
                                                 height: _appBarHeight
                                             ),