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/allowlists/allowlists.go

@@ -1347,4 +1347,8 @@ var (
 	ProdMixedBuildsEnabledList = []string{
 		"com.android.adbd",
 	}
+
+	// Staging builds should be entirely prod, plus some near-ready ones. Add the
+	// new ones to the first argument as needed.
+	StagingMixedBuildsEnabledList = append([]string{}, ProdMixedBuildsEnabledList...)
 )