Implement trigger postinstall API

Test: adb shell update_engine_client --trigger_postinstall=system
Bug: 377557752
Change-Id: Ieb28e86bd979502c5b208cd8df917f3e49a50f0b
diff --git a/payload_consumer/payload_metadata.cc b/payload_consumer/payload_metadata.cc
index d2e42f0..649d9be 100644
--- a/payload_consumer/payload_metadata.cc
+++ b/payload_consumer/payload_metadata.cc
@@ -154,7 +154,7 @@
 }
 
 ErrorCode PayloadMetadata::ValidateMetadataSignature(
-    const brillo::Blob& payload,
+    const std::string_view payload,
     const string& metadata_signature,
     const PayloadVerifier& payload_verifier) const {
   if (payload.size() < metadata_size_ + metadata_signature_size_)