update_engine: fix cow estimation error am: 2c8189d3c2 am: a5151ab952

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3019785

Change-Id: I9c630d5036a6321ce8b623cdd8d598c87db4ff4f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>