Add gki.prevent_downgrade_*

Add ro.build.ab_update.gki.prevent_downgrade_{version,spl} for
update_engine to determine whether downgrade in kernel version or SPL is
considered an error or not.

Bug: 162623577
Test: update_engine_unittest
Test: apply OTA

Change-Id: If602924d50a2d5cfb3c256b82491c413a9d39f9d
4 files changed