libprocessgroup: Use cgroup.kill

By using cgroup.kill we don't need to read cgroup.procs at all for
SIGKILLs, which is more efficient and should help reduce CPU contention
and cgroup lock contention. Fallback to cgroup.procs if we encounter an
error trying to use cgroup.kill, but if cgroup.kill fails it's likely
that cgroup.procs will too.

Bug: 239829790
Test: atest StagedRollbackTest:com.android.tests.rollback.host.StagedRollbackTest#testNativeWatchdogTriggersRebootlessApexRollback
Change-Id: I9da67efd00af49b7b4b502fb742c1095d5c7b9e9