libprocessgroup: Add a function to remove only empty process groups am: 6761b6dc

Original change: https://googleplex-android-review.googlesource.com/c/platform/system/core/+/18090770



Change-Id: I45e4f502cdf958c26b22370b43c7c8215c18fde9
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>