Improve diagnostics in ART script tools/teardown-buildbot-device.sh.

In case of unmounting failure, show the list of open files
in the directory that we tried to unmount.

Also refactor the logic of unmounting and removing directories
under the chroot dir.

Test: art/tools/setup-buildbot-device.sh; sh -x art/tools/teardown-buildbot-device.sh
Bug: 34729697
Change-Id: I22f9414c3a6e2ae37f90a7eeff058e2c4252dbbf