[Android] Retrieve the specific xwalk application template version.

This tool is added for user to conveniently retrieve the xwalk application
tempalte package, it covers the following functions.
1 Retrieve the url address of the specific crosswalk package version.
2 Download the crosswalk package and extract it to the destination directory.
3 Extract the xwalk application template from the unpacked crosswalk package.
Merge pull request #6 from ShDavidliu/app

[Android] Retrieve the specific xwalk application template version.