Allow overriding COW version in host-side cow_converter am: e7ca36910b am: a2ebb243c1

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2886532

Change-Id: I98176808b01133e15e5b9f38208785193f6f9003
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>