Apply media padding to Gallery video demo (#13847)

Applies horizontal safe area insets to the video demo in the Gallery.
This is to support the iPhone X sensor housing notch and other similarly
creative display features when in landscape orientation.

examples/flutter_gallery/lib/demo/video_demo.dart

@@ -85,7 +85,10 @@ class VideoCard extends StatelessWidget {
       Navigator.of(context).push(route);
     }
 
-    return new Card(
+    return new SafeArea(
+      top: false,
+      bottom: false,
+      child: new Card(
         child: new Column(
           children: <Widget>[
             new ListTile(title: new Text(title), subtitle: new Text(subtitle)),
@@ -95,6 +98,7 @@ class VideoCard extends StatelessWidget {
             ),
           ],
         ),
+      ),
     );
   }
 }