New version app update

1. Download, Install the new version app.
2. the old data maintenance

Solution:
1. It seems that google play store itself can solve the app downloading and updating problems. So we don't need unecessary logic code. You know, google play is banned in china so we alwalys need write our own logic for checking version, downloading and installing the app.

2. In this app, I just use the sharepreference to store some unimportant data. And SharePreference will be kept automatically during the update process. So no need some changes.