Skip ANR for processes that have been killed

If a controller is attached and decides to kill a process after an ANR,
other ANR reports for that app that are queued up won't be handled until
after the app has died.  This can create a report without the relevant
callstacks because the app is dead by the time the traces are dumped.
Since the trace file is global, the traces recorded for the first ANR
are overwritten, leaving us with no clue as to what happened.

After the app has been killedByAm, it is not interesting to handle ANRs
for that app until it is started again, so killedByAm can be used to
filter out these spurious reports.

Change-Id: I34ba790f6d29d563c819dc2f6ac71a3c8955bb76