Improve atomiticy of update checkpointing am: c80d39b63b

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

Change-Id: I7e945b658e9b167b751b3185a8701c8e25151bf9
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>