cow estimation: use old partition size am: 1ab6415902 am: 5dab47b25b

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3025808

Change-Id: Icb5d651c2cd41ddfe06b45df7bf8d3eadd434f8d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>