Retry: Allow modules to override --error-when-new UnflaggedApi

Some `java_sdk_library` and `droidstubs` modules already specify
`--error UnflaggedApi` but they are currently ignored because they are
added before the `--error-when-new UnflaggedApi` automatically added by
Soong and Metalava uses the last setting. That means adding
`--error-when-new UnflaggedApi` in change https://r.android.com/3248013
actually reduced the severity of the issue for those modules making it
more likely that they would be missed.

This change only adds the `--error-when-new UnflaggedApi` if it does
not already appear in the metalava command.

Bug: 361582214
Test: m checkapi
Change-Id: I7228bf9794b1c7e27d7acf7a461956b30f3335f5
diff --git a/java/droidstubs.go b/java/droidstubs.go
index 137ec92..6bcdf85 100644
--- a/java/droidstubs.go
+++ b/java/droidstubs.go
@@ -970,10 +970,14 @@
 		d.apiLintReport = android.PathForModuleOut(ctx, Everything.String(), "api_lint_report.txt")
 		cmd.FlagWithOutput("--report-even-if-suppressed ", d.apiLintReport) // TODO:  Change to ":api-lint"
 
-		// Make sure that existing UnflaggedApi issues are reported as warnings but issues in
-		// new/changed code are treated as errors by the Build Warnings Aye Aye Analyzer in Gerrit.
+		// If UnflaggedApi issues have not already been configured then make sure that existing
+		// UnflaggedApi issues are reported as warnings but issues in new/changed code are treated as
+		// errors by the Build Warnings Aye Aye Analyzer in Gerrit.
 		// Once existing issues have been fixed this will be changed to error.
-		cmd.Flag("--error-when-new UnflaggedApi")
+		// TODO(b/362771529): Switch to --error
+		if !strings.Contains(cmd.String(), " UnflaggedApi ") {
+			cmd.Flag("--error-when-new UnflaggedApi")
+		}
 
 		// TODO(b/154317059): Clean up this allowlist by baselining and/or checking in last-released.
 		if d.Name() != "android.car-system-stubs-docs" &&