Kill stray processes spawn by some java.lang.Process related tests.

The following tests execute "sleep" commands, but did not destroy the
corresponding processes:
- org.apache.harmony.tests.java.lang.ProcessManagerTest#testSleep
- org.apache.harmony.tests.java.lang.ProcessTest#test_exitValue

These processes would eventually die (at the end of the sleep period),
but in the case of chroot-based on-device testing on the ART Buildbot,
these stray processes would prevent the unmounting of the /dev
filesystem in the chroot directory by the tear-down script. Explicitly
kill these processes at the end of these tests.

Test: art/tools/run-libcore-tests.sh --mode=device --variant=X64 && art/tools/teardown-buildbot-device.sh
Bug: 34729697
Change-Id: I92c83fa442cd916ce9cce268b784a4a974758132