Kill the entire child process group when cancelling postinstall am: 866d470c1f am: 9bb7bae4e0

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

Change-Id: I5a2c285a75b104fe265f5fce47b64c3cf53fc7b6
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>