init: change kill order and fix error reporting in KillProcessGroup()

First kill the process group before killing the cgroup to catch
the hopeful case that killing the cgroup becomes a no-op as all of its
processes have already been killed.

Do not report an error if kill fails due to ESRCH, as this happens
often when reaping processes due to the order in which we call
waitpid() and kill().

Do not call killProcessGroup in libprocessgroup if we have already
successfully killed and removed a process group.

Bug: 36661364
Bug: 36701253
Bug: 37540956

Test: Reboot bullhead
Test: Start and stop services
Test: Init unit tests
Change-Id: I172acf0f8e00189f910f865f4635a7b1782fc7e3