BrowseErrorFragment: fix NPE of not attached BackgroundManager

BrowseErrorFragment also shows Background for selected item.
Need attach BackgroundManager to window.

b/22796403

Change-Id: I80bbaf48c85a318fdd0e07d4efd3d1b72876c03d

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

@@ -39,6 +39,18 @@ public class BrowseErrorActivity extends Activity
         testError();
     }
 
+    @Override
+    public void onAttachedToWindow() {
+        super.onAttachedToWindow();
+        BackgroundHelper.attach(this);
+    }
+
+    @Override
+    public void onStop() {
+        BackgroundHelper.release(this);
+        super.onStop();
+    }
+
     private void testError() {
         mErrorFragment = new ErrorFragment();
         getFragmentManager().beginTransaction().add(R.id.main_frame, mErrorFragment).commit();