Make checkpointing atomic am: 5eece04aaf

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2597085

Change-Id: I677c2ea84d2e266b1b174c8663e885372839eeca
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>