Speed up chroot process lookup in ART Buildbot tear-down script.

Instead of executing "adb shell readlink" on every file matching the
search pattern ("/proc/*/root") to find symbolic links to the chroot
dir, filter the output of "adb shell ls -ld" and iterate on that
sublist directly. Reduces the script execution time from 53s to 3s in
local testing.

Test: Rely on the ART Buildbot
Bug: 34729697
Change-Id: Ib2ce38b37cdcec43c2ea966c89d13de794e91380