Add a --verify_only flag, which verifies the payload am: 63b39110c1 am: 594ea45652

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1621760

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I3c4b0f59eeff68d81ce7272164debf1179a909ee