Refactor for new libsnapshot_cow API. am: 8144b25bba

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

Change-Id: I910516ef74311d85c37e339d65e6e414cdec77ec
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>