update_engine: fix cow estimation error am: 2c8189d3c2

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

Change-Id: If74de19294bd66187345f13dead532be7b620f17
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>