Fix problem:

- The property timeToArrive can't be updated in the LongListSelector after RefreshBusTime()
- The property timeToArrive of newly added bus-station cannot be updated in the LongListSelector.
解決了MainPage.xaml中的LongListSelector中的某些row(比如說，中間的橘2)的timeToArrive無法更新。以及新增的巴士的timeToArrive無法更新。