Add a library function for performing a cow dry run am: 4ab946d067 am: df5cb43bf3

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1590431

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I6c49836a30de2a6c855fe43084192997da44c8df