Improve atomiticy of update checkpointing am: c80d39b63b am: afc3c0d993

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2803169

Change-Id: If77d215d8e27b444614005c1244bd7def0931e5d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>