Kill chroot processes at the end of the ART Buildbot tear-down script.

If we catch processes from binaries located under the chroot
directory still running at the end of the tear-down step, kill
them.

Also refactor the tear-down script a bit.

Test: Rely on the ART Buildbot
Bug: 34729697
Change-Id: I49d11f4eb4e7251d1b95cc2d1fbe6fa46af21ac8
1 file changed