Reject SPL downgrade for locked bootlaoders

Test: th
Bug: 270459308
Change-Id: I46ce6c59afed28377fa1513f9eb7e098755cde40
diff --git a/payload_consumer/delta_performer.h b/payload_consumer/delta_performer.h
index 7365519..12238a7 100644
--- a/payload_consumer/delta_performer.h
+++ b/payload_consumer/delta_performer.h
@@ -311,7 +311,7 @@
   // Check if partition `part_name` is a dynamic partition.
   bool IsDynamicPartition(const std::string& part_name, uint32_t slot);
 
-  void CheckSPLDowngrade();
+  bool CheckSPLDowngrade();
 
   // Update Engine preference store.
   PrefsInterface* prefs_;