[Android] Fork the target for sample app from app template

Sample application should not share the same code with
application template for web developer.
We might want to demonstrate some xwalk's feature in
sample but not in app template.

BUG=https://github.com/crosswalk-project/crosswalk/issues/725
Merge pull request #731 from wang16/app_demo

[Android] Fork the target for demo from app template