Prefer vendor prop when fetching ab partition list am: 996766a950 am: 16ff6de8e1

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2597086

Change-Id: If8883d1d171e47d04af3636e9d0d9ca1113d7add
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>