cow estimation: use old partition size am: 1ab6415902

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

Change-Id: I2301bdc08f54563a0e87b2fa411164b2987c1974
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>