libprocessgroup: Add a function to remove only empty process groups am: 6761b6dc am: f98610b8

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



Change-Id: Icbd9e3ed276fdf231b82d9aa2d9b2c94d534c41b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>