Kill the entire child process group when cancelling postinstall am: 866d470c1f am: 31e74e6d13

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

Change-Id: I6063f0bc5411b778a399cd3767b9de6b348faca0
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>