Add bazel staging mode to soong build.

This is to use bazel to build targets that are being prepared for an
incipient release to the prod mode allowlist.

Bug: 254265047
Test: m nothing
Test: m nothing --bazel-mode-dev
Test: m nothing --bazel-mode-staging
Change-Id: Ic78a59cf51dba83ef1ac26483586560ea9b24aaf
diff --git a/ui/build/soong.go b/ui/build/soong.go
index 28c6ec9..c87f993 100644
--- a/ui/build/soong.go
+++ b/ui/build/soong.go
@@ -260,6 +260,9 @@
 	if config.bazelDevMode {
 		mainSoongBuildExtraArgs = append(mainSoongBuildExtraArgs, "--bazel-mode-dev")
 	}
+	if config.bazelStagingMode {
+		mainSoongBuildExtraArgs = append(mainSoongBuildExtraArgs, "--bazel-mode-staging")
+	}
 
 	mainSoongBuildInvocation := primaryBuilderInvocation(
 		config,