update_engine: Change imgdiff/imgpatch to a dummy puffdiff/puffpatch am: 49fdb09d1f am: 899d48abf5 am: fde9a7dab3 am: a162d7b044
am: 9ca33d62a9

Change-Id: Iab94f1931fc474f4528bd800c175ad3cc1723bdb