init: always kill oneshot services' process groups.

~2007 a change was added that would allow oneshot services to
daemonize by not killing their process group.  This was a hack at the
time, and should certainly not be needed now.  I've resisted removing
the behavior however, as it hadn't caused any issues.

Recently, it was detected that the cgroups that these processes belong
to, would exist forever and therefore leak memory.  Instead of simply
removing the cgroups when empty, this provides a good opportunity to
do the right thing and fix this behavior once and for all.

The new (correct) behavior only happens for devices with vendor images
built for Android R or later.  Init will log a warning to dmesg when
it detects this difference in behavior has occurred.

Bug: 144545923
Test: boot CF/Coral and see no difference in behavior.
Test: boot CF with a service that daemonizes and see the warning.
Change-Id: I333a2e25a541ec0114ac50ab8ae7f1ea3f055447