[Android] Fix build error for xwalk core library

It's caused by change introduced by rebasing to 34.0.1847.45.
Fix by adding the necessary files to the list in
generate_xwalk_core_library.py.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1201
Merge pull request #1712 from wang16/fix_cordova

[Android] Fix build error for xwalk core library