[Android] Fix xwalk_runtime_lib_apk build failed on canary.

Rewrite the "prepare_xwalk_app_template" target with python
due to a bug in gyp system, that "copies" action doesn't respect
the "dependencies".

BUG=https://github.com/crosswalk-project/crosswalk/issues/771
Merge pull request #772 from junmin-zhu/gyp

[Android] Fix xwalk_runtime_lib_apk build failed on canary.