Revert "[Android] Remove all java source files in xwalk_core_library"

This reverts commit a1a971bfeefe6fc3b314a488bd20e667d010ae14.
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.