Append previous_api to "--migrate-nullness" option

"--previous-api" option was not recommended after Metalava was
changed on how to consume the api file args.

Test: m -j api-stubs-docs
Bug: b/78245848
Change-Id: I1054173e11b4d47942dda0b95193277758aa8193
diff --git a/java/droiddoc.go b/java/droiddoc.go
index f90c548..104e46d 100644
--- a/java/droiddoc.go
+++ b/java/droiddoc.go
@@ -950,9 +950,8 @@
 		previousApi := ctx.ExpandSource(String(d.properties.Metalava_previous_api),
 			"metalava_previous_api")
 		*implicits = append(*implicits, previousApi)
-		flags += " --previous-api " + previousApi.String()
 
-		flags += " --include-annotations --migrate-nullness"
+		flags += " --include-annotations --migrate-nullness " + previousApi.String()
 
 		d.annotationsZip = android.PathForModuleOut(ctx, ctx.ModuleName()+"_annotations.zip")
 		*implicitOutputs = append(*implicitOutputs, d.annotationsZip)