Reapply "libprocessgroup: Poll on cgroup.events" am: 4928b6ef am: 24e2480b am: e7655425

Original change: https://android-review.googlesource.com/c/platform/system/core/+/2876955



Change-Id: Ie8157058d5d0cea4573b323408ba75c02320955d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>