Verify if pid actually killed for processes with open files

Its possible for vold to read a pid from procfs, the pid is killed
externally and then vold tries to kill it. In this scenario, we sleep
for 5s without needing it. Verify the return value from the kill syscall
and validate that the pid was killed, if the pid didn't exist at the
moment of the kill call, then don't count the pid as being killed.

Test: Boots successfully
Bug: 307801020
Change-Id: Ie127108b85be7249cf8b2881f4917d653d032186