[Android] Integrate xpk support in packaging tools.

xpk is the format of application package,
which is managed by xwalk runtime.
It should be the default input of packaging tools.
The format of xpk is defined here:
https://github.com/crosswalk-project/crosswalk-website/wiki/Crosswalk-package-management.

Usage: "python make_apk.py --xpk=the/path/to/xpk/file"

BUG=https://github.com/crosswalk-project/crosswalk/issues/738