Apply media padding to Contacts demo (#13628)

Applies horizontal safe area insets to the Contacts 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/contacts_demo.dart

@@ -20,6 +20,9 @@ class _ContactCategory extends StatelessWidget {
       ),
       child: new DefaultTextStyle(
         style: Theme.of(context).textTheme.subhead,
+        child: new SafeArea(
+          top: false,
+          bottom: false,
           child: new Row(
             crossAxisAlignment: CrossAxisAlignment.start,
             children: <Widget>[
@@ -29,9 +32,10 @@ class _ContactCategory extends StatelessWidget {
                 child: new Icon(icon, color: themeData.primaryColor)
               ),
               new Expanded(child: new Column(children: children))
-          ]
-        )
-      )
+            ],
+          ),
+        ),
+      ),
     );
   }
 }