Reject SPL downgrade for locked bootlaoders am: bdba354291 am: 076a9be63f

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

Change-Id: I18b0d95348c13c69ced267b3d6cffc4753ebf7a9
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>