Refactor for new libsnapshot_cow API. am: 8144b25bba am: 0c4395cf73

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2583169

Change-Id: I1628fc3b64244c76a7c2f08d721ce1871e0d4464
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>