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/build.go b/ui/build/build.go
index 2022e50..b9bd898 100644
--- a/ui/build/build.go
+++ b/ui/build/build.go
@@ -112,9 +112,19 @@
 // checkBazelMode fails the build if there are conflicting arguments for which bazel
 // build mode to use.
 func checkBazelMode(ctx Context, config Config) {
-	if config.bazelProdMode && config.bazelDevMode {
+	count := 0
+	if config.bazelProdMode {
+		count++
+	}
+	if config.bazelDevMode {
+		count++
+	}
+	if config.bazelStagingMode {
+		count++
+	}
+	if count > 1 {
 		ctx.Fatalln("Conflicting bazel mode.\n" +
-			"Do not specify both --bazel-mode and --bazel-mode-dev")
+			"Do not specify more than one of --bazel-mode and --bazel-mode-dev and --bazel-mode-staging ")
 	}
 }