Kill the entire child process group when cancelling postinstall am: 866d470c1f

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2802553

Change-Id: I6decfb7a3f1585897416956fc126faadb2e17bdf
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>