SupportLenabackDemo: Make StepFragment public

If for any reason e.g. BrowseActivty crash or rotate screen on phone,
system needs a public empty parameter constructor to restore
fragment stack.

Change-Id: I4c001b4b95a1958ff7f2cb42688582c8261a71e0

samples/SupportLeanbackDemos/src/com/example/android/leanback/MainActivity.java

@@ -42,7 +42,7 @@ public class MainActivity extends Activity {
         GuidedStepFragment.add(getFragmentManager(), mGuidedStepFragment);
     }
 
-    private static class StepFragment extends GuidedStepFragment {
+    public static class StepFragment extends GuidedStepFragment {
         @Override
         public Guidance onCreateGuidance(Bundle savedInstanceState) {
             String title = getString(R.string.main_title);