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

android/bazel_handler.go

@@ -386,6 +386,12 @@ func NewBazelContext(c *config) (BazelContext, error) {
 		for _, enabledProdModule := range allowlists.ProdMixedBuildsEnabledList {
 			enabledModules[enabledProdModule] = true
 		}
+	case BazelStagingMode:
+		modulesDefaultToBazel = false
+		for _, enabledStagingMode := range allowlists.StagingMixedBuildsEnabledList {
+			enabledModules[enabledStagingMode] = true
+
+		}
 	case BazelDevMode:
 		modulesDefaultToBazel = true