Only prevent boot image downgrade if sysprop set. am: 126d13ee33 am: 6ba6aad026 am: c24ae820c1

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1433516

Change-Id: I5cef87a8953e7315af329c1c06598b8390bd32a5