Rename variable containing Core Java libraries in build/apex/Android.bp.

Rename `libcore_target_java_libs` as `libcore_java_libs`, as that
variable is used for both device and host APEX packages.

Test: mmma art/build/apex
Bug: 113373927
Change-Id: Iad2838437aacc3c267f140c036c6dfcd479bec6c