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

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

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: Ib7a2c3e37b94c1bf7e07954ad6821ea3b814341d