Gardening M37 : Fix make_apk and make_apk_test

- Make sure to reflect the fact that theme.xml was moved.
- UI strings are now generated slightly differently, our scripts
need to be adapted.
- Rename the string generation target in GYP to something with
"internal" to make it clear.
here should be `xwalk_core_internal_java` since all resources are in `xwalk_core_internal` now.