Reject SPL downgrade for locked bootlaoders am: bdba354291

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2457389

Change-Id: I643b483ad545276a9f709926c3dc207a52f52a56
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>