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/config.go b/ui/build/config.go
index f6f5b46..cde8d5d 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -100,8 +100,9 @@
 
 	pathReplaced bool
 
-	bazelProdMode bool
-	bazelDevMode  bool
+	bazelProdMode    bool
+	bazelDevMode     bool
+	bazelStagingMode bool
 
 	// Set by multiproduct_kati
 	emptyNinjaFile bool
@@ -721,6 +722,8 @@
 			c.bazelProdMode = true
 		} else if arg == "--bazel-mode-dev" {
 			c.bazelDevMode = true
+		} else if arg == "--bazel-mode-staging" {
+			c.bazelStagingMode = true
 		} else if len(arg) > 0 && arg[0] == '-' {
 			parseArgNum := func(def int) int {
 				if len(arg) > 2 {
@@ -1115,7 +1118,7 @@
 }
 
 func (c *configImpl) BazelBuildEnabled() bool {
-	return c.bazelProdMode || c.bazelDevMode
+	return c.bazelProdMode || c.bazelDevMode || c.bazelStagingMode
 }
 
 func (c *configImpl) StartRBE() bool {