Improve atomiticy of update checkpointing am: c80d39b63b

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

Change-Id: I8f2da9158bb4b2423041b15c6a93b8c133b78dcb
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>