[Android] Generate source package of xwalk core library

Developers might want to do some customization based on
xwalk core library.
Source package of xwalk core library could meet the
requirement.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2587
Merge pull request #2457 from wang16/core_library_src

[Android] Generate source package of xwalk core library