Reapply "libprocessgroup: Poll on cgroup.events"

This reverts commit a72332f9.

This change was originally reverted due to a bug in the child commit
which caused process group kills not to occur when using cgroup.kill.
Now that has been fixed, bring back this change with the fixed child.

Bug: 301871933
Change-Id: Ia6c74d9b67a8c88aec4812ac4655646934e0d189