Use swapCursor(null) instead of changeCursor(null)

The later closes the old cursor while the former doesn't.
LoaderManager takes care of closing cursors, so no need to close
cursors by ourselves.

Change-Id: Ib652e6877dcfc1fea51ffc862254eaeed451cdfa

src/com/android/email/activity/MessageListFragment.java

@@ -365,7 +365,7 @@ public class MessageListFragment extends ListFragment
         stopLoaders();
         onDeselectAll();
         if (mListAdapter != null) {
-            mListAdapter.changeCursor(null);
+            mListAdapter.swapCursor(null);
         }
         setListShownNoAnimation(false);
     }