Implement the topology sort in CreateCowMergeOperation am: 31ad11922c am: 43577a4992 am: 12d4fdc3d8

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1396952

Change-Id: Ief84591efd1fc261d9524b267c1573dc6353a75d