Only prevent boot image downgrade if sysprop set. am: 126d13ee33 am: 6ba6aad026 am: c24ae820c1 am: 0030a0f247

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

Change-Id: I9ce13d6f51a4b747992dfd07e5d3784f95625f57