[Android] Fix that R.class in core library is set with incorrect value

The R.java chromium build process for apk generated in the
trival target based on xwalk_core_library is with incorrect
value set.

However, what we needed is the declare class from resource map
instead of the one with value. To fix this, build
xwalk_core_library_empty target as test apk to exclude resource
generation, besided add resource map as additional src to include
the correct declaration of R.java.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1577