Add support for java_defaults modules

Test: java_test.go
Change-Id: I6dba1671c7eb019183af94bb7b10810296740101
diff --git a/android/mutator.go b/android/mutator.go
index 86a5b85..c8f3e8f 100644
--- a/android/mutator.go
+++ b/android/mutator.go
@@ -75,7 +75,7 @@
 		ctx.TopDown("load_hooks", loadHookMutator).Parallel()
 	},
 	registerPrebuiltsPreArchMutators,
-	registerDefaultsPreArchMutators,
+	RegisterDefaultsPreArchMutators,
 }
 
 var preDeps = []RegisterMutatorFunc{