[Android] Remove all java source files in xwalk_core_library

In previous commit a3cd81484.
Some chromium generated source files including R.java and
NativeLibraries.java are still copied into xwalk_core_library.

With this commit, an empty apk target xwalk_core_library_empty
which only depends on xwalk_core_java with no extra code will
be used to get the classes for the chromium generated java codes.
And package them into a jar file.