Improve atomiticy of update checkpointing am: c80d39b63b am: f28998e195

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

Change-Id: I007d9c60d9fa9b8922fb1333740190560424e8cf
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>