Fix crash when recreating views of reused MyScheduleFragments

Fix the following crash when recreating views for reused fragments in the "My Schedule" screen.

```
java.lang.IllegalStateException: Cannot add header view to list -- setAdapter has already been called.
  at android.widget.ListView.addHeaderView(ListView.java:265)
  at android.widget.ListView.addHeaderView(ListView.java:294)
  at com.google.samples.apps.iosched.ui.MyScheduleActivity.onFragmentViewCreated(MyScheduleActivity.java:371)
  at com.google.samples.apps.iosched.ui.MyScheduleFragment.onViewCreated(MyScheduleFragment.java:62)
  …
```

Crashes on a test device with Android 4.3 when showing 3 conference days. Seems to have been fixed in newer Android versions.