message: Make "toProtoKeySerialization" private.
  This isn't used outside and should be private.
diff: @@ -487,7 +487,7 @@ public final class KeysetHandle {
  .build();
  }
  - static ProtoKeySerialization toProtoKeySerialization(Keyset.Key protoKey) {
  + private static ProtoKeySerialization toProtoKeySerialization(Keyset.Key protoKey) {
  int id = protoKey.getKeyId();
  @Nullable
  Integer idRequirement = protoKey.getOutputPrefixType() == OutputPrefixType.RAW ? null : id;
  