[animations] Refactor Radio widgets in example

Refactors the `shared_axis_transition.dart` example to use a new `RadioGroup` widget. This removes the deprecated `groupValue` and `onChanged` properties from each individual `Radio` button

Author: alexskobozev

packages/animations/example/lib/shared_axis_transition.dart

@@ -76,34 +76,28 @@ class _SharedAxisTransitionDemoState extends State<SharedAxisTransitionDemo> {
               ),
             ),
             const Divider(thickness: 2.0),
-            Row(
-              mainAxisAlignment: MainAxisAlignment.center,
-              children: <Widget>[
-                Radio<SharedAxisTransitionType>(
-                  value: SharedAxisTransitionType.horizontal,
-                  groupValue: _transitionType,
-                  onChanged: (SharedAxisTransitionType? newValue) {
-                    _updateTransitionType(newValue);
-                  },
-                ),
-                const Text('X'),
-                Radio<SharedAxisTransitionType>(
-                  value: SharedAxisTransitionType.vertical,
-                  groupValue: _transitionType,
-                  onChanged: (SharedAxisTransitionType? newValue) {
-                    _updateTransitionType(newValue);
-                  },
-                ),
-                const Text('Y'),
-                Radio<SharedAxisTransitionType>(
-                  value: SharedAxisTransitionType.scaled,
-                  groupValue: _transitionType,
-                  onChanged: (SharedAxisTransitionType? newValue) {
-                    _updateTransitionType(newValue);
-                  },
-                ),
-                const Text('Z'),
-              ],
+            RadioGroup<SharedAxisTransitionType>(
+              groupValue: _transitionType,
+              onChanged: (SharedAxisTransitionType? newValue) {
+                _updateTransitionType(newValue);
+              },
+              child: Row(
+                mainAxisAlignment: MainAxisAlignment.center,
+                children: <Widget>[
+                  Radio<SharedAxisTransitionType>(
+                    value: SharedAxisTransitionType.horizontal,
+                  ),
+                  const Text('X'),
+                  Radio<SharedAxisTransitionType>(
+                    value: SharedAxisTransitionType.vertical,
+                  ),
+                  const Text('Y'),
+                  Radio<SharedAxisTransitionType>(
+                    value: SharedAxisTransitionType.scaled,
+                  ),
+                  const Text('Z'),
+                ],
+              ),
             ),
           ],
         ),