Fix cow size estimation of --vabc-none am: bfa10418a2

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2494986

Change-Id: I603e517a793f320c1f87de750531683bcedde731
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>