update_engine: fix checkpointing am: 37f212d9fe

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3218418

Change-Id: Ib666221b6b1939087c553e950049a150be1d4c62
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>