Don't use dense dropdown buttons in the Gallery date/time picker demo (#21693)

examples/flutter_gallery/lib/demo/material/date_and_time_picker_demo.dart

@@ -181,15 +181,16 @@ class _DateAndTimePickerDemoState extends State<DateAndTimePickerDemo> {
                   });
                 },
               ),
+              const SizedBox(height: 8.0),
               InputDecorator(
                 decoration: const InputDecoration(
                   labelText: 'Activity',
                   hintText: 'Choose an activity',
+                  contentPadding: EdgeInsets.zero,
                 ),
                 isEmpty: _activity == null,
                 child: DropdownButton<String>(
                   value: _activity,
-                  isDense: true,
                   onChanged: (String newValue) {
                     setState(() {
                       _activity = newValue;

examples/flutter_gallery/test/accessibility_test.dart

@@ -58,7 +58,7 @@ void main() {
       await tester.pumpWidget(MaterialApp(home: DateAndTimePickerDemo()));
       expect(tester, meetsGuideline(androidTapTargetGuideline));
       handle.dispose();
-    }, skip: true); // https://github.com/flutter/flutter/issues/21578
+    });
 
     testWidgets('dialog_demo', (WidgetTester tester) async {
       final SemanticsHandle handle = tester.ensureSemantics();