Parallelize singleton execution

Bug: 281536768
Test: manual, presubmits
Change-Id: I57fdc76ba6b277e88e196b506af87127a530fd37
diff --git a/rust/doc.go b/rust/doc.go
index fe3581b..6970d79 100644
--- a/rust/doc.go
+++ b/rust/doc.go
@@ -19,7 +19,7 @@
 )
 
 func init() {
-	android.RegisterSingletonType("rustdoc", RustdocSingleton)
+	android.RegisterParallelSingletonType("rustdoc", RustdocSingleton)
 }
 
 func RustdocSingleton() android.Singleton {