Use flag names with merge_csv.py.

Bug: 145132366
Test: diff -w out/soong/hiddenapi/hiddenapi-greylist.csv
Change-Id: If63c8ae5eda48e49bf58a338bf08f0f2d4fea3cf
diff --git a/java/hiddenapi_singleton.go b/java/hiddenapi_singleton.go
index 3243e43..7850193 100644
--- a/java/hiddenapi_singleton.go
+++ b/java/hiddenapi_singleton.go
@@ -294,9 +294,8 @@
 
 	rule.Command().
 		BuiltTool(ctx, "merge_csv").
-		Inputs(metadataCSV).
-		Text(">").
-		Output(outputPath)
+		FlagWithOutput("--output=", outputPath).
+		Inputs(metadataCSV)
 
 	rule.Build(pctx, ctx, "hiddenAPIGreylistMetadataFile", "hiddenapi greylist metadata")