Parallelize singleton execution

Bug: 281536768
Test: manual, presubmits
Change-Id: I57fdc76ba6b277e88e196b506af87127a530fd37
diff --git a/rust/project_json.go b/rust/project_json.go
index fe259d6..40aa7c7 100644
--- a/rust/project_json.go
+++ b/rust/project_json.go
@@ -74,7 +74,7 @@
 }
 
 func init() {
-	android.RegisterSingletonType("rust_project_generator", rustProjectGeneratorSingleton)
+	android.RegisterParallelSingletonType("rust_project_generator", rustProjectGeneratorSingleton)
 }
 
 // sourceProviderVariantSource returns the path to the source file if this