Avoid crashes in a single app from causing cascading test failures

This should work properly for crashes.  It currently doesn't do the right
thing for ANRs since, in a lot of cases, they seem to happen asynchronously
_after_ the testcase has ended.  Will try to improve that behavior with a
subsequent change.

Bug: 6128185
Change-Id: Ie535141e879062c11ee7108b37d282a33a5b5eef