Add support for flattened Runtime APEX to Make rule `standalone-apex-files`.

Adjust the source of the Runtime APEX libraries copied to the
/system/lib(64) directory sync'd to the target, so that this rule
works both for non-flattened and flattened APEXes.

Test: ART chroot-based on-device testing using the master-art branch
Bug: 121117762
Change-Id: I9f796e58c22c6346d11416742ed8b81baa712c6a