Display users of the chroot dir in the ART Buildbot tear-down script.

This should help debug issues with files still open / processes still
running at the end of a run.

Also fix the use of toybox's lsof when unmounting a filesystem within
the chroot fails.

Test: Rely on the ART Buildbot
Bug: 34729697
Change-Id: I6dc46724b2fd0b29df072b54414369c7909d453d