Use flattened APEX packages in ART Buildbot targets.

This is to simplify the use of APEX packages on the ART Buildbot (and
later on Golem).

Test: ART chroot-based on-device testing using the master-art branch
Bug: 121117762
Change-Id: I9973adcbae604d54f4486e713bca2c742ead4526
1 file changed