send SIGTERM only once

- Processes capturing SIGTERM signal will not terminate and
  killProcessGroup will timeout after retry. This gives 200ms
  of waiting per such process. Change it to retry only once as
  SIGTERM will keep failing for such process.

bug: 36696408
Test: check time spent in SIGTERM for reboot
Change-Id: I9b0843614598dd3f97a50b21352046297ed70b6b