Add support for java_defaults modules

Test: java_test.go
Change-Id: I6dba1671c7eb019183af94bb7b10810296740101
diff --git a/android/defaults.go b/android/defaults.go
index 3ec3ff4..4bf872e 100644
--- a/android/defaults.go
+++ b/android/defaults.go
@@ -117,7 +117,7 @@
 	}
 }
 
-func registerDefaultsPreArchMutators(ctx RegisterMutatorsContext) {
+func RegisterDefaultsPreArchMutators(ctx RegisterMutatorsContext) {
 	ctx.BottomUp("defaults_deps", defaultsDepsMutator).Parallel()
 	ctx.TopDown("defaults", defaultsMutator).Parallel()
 }