Allow defaults modules to have defaults

Allow chaining defaults modules by making Defaults be a Defaultable.

Change-Id: I4ba86c07f7aad9c396ed33d55fe95d1fb78e487d
diff --git a/android/arch.go b/android/arch.go
index 13fcb29..d6b2836 100644
--- a/android/arch.go
+++ b/android/arch.go
@@ -26,7 +26,7 @@
 
 func init() {
 	RegisterBottomUpMutator("defaults_deps", defaultsDepsMutator)
-	RegisterTopDownMutator("defaults", defaultsMutator)
+	RegisterBottomUpMutator("defaults", defaultsMutator)
 
 	RegisterBottomUpMutator("arch", ArchMutator)
 }