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

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

Change-Id: I6fc42e2c884a3c9a7d00de861eedf6423b6cc7ca
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>