add killProcessGroupOnce

- killProcessGroup retries if it fails while
  killProcessGroupOnce only tries once.
- useful for sending signal like SIGTERM once.

bug: 36696408
Test: will add usage to init

(cherry picked from commit a88e425e0f5502062c7f3088d53b1b0ce8ef803e)

Change-Id: Ie8eb338ee919b499948baf2a220e584b54293a89