[Android] Remove xwalk_core_library_java.jar from the AAR package.

This fixes another regression inherited from the previous version of the
script, and is also required for XWALK-1930: xwalk_core_library_java.jar
is not supposed to be packaged twice, it should be present as
classes.jar and not be part of jni/.
Merge pull request #2421 from rakuco/even-more-aar-packaging-fixes

[Android] Remove xwalk_core_library_java.jar from the AAR package.