Allow defining neverallow in packages other than android.

This exposes test func on ValueMatcher and helpers relevant for testing neverallows.

Bug: 153333044
Test: m
Change-Id: Ied07cd33afa537f9d7aa3c33e59f4ac985901a0f
diff --git a/android/mutator.go b/android/mutator.go
index a46d4be..10a815a 100644
--- a/android/mutator.go
+++ b/android/mutator.go
@@ -103,7 +103,7 @@
 	registerPathDepsMutator,
 	RegisterPrebuiltsPostDepsMutators,
 	RegisterVisibilityRuleEnforcer,
-	registerNeverallowMutator,
+	RegisterNeverallowMutator,
 	RegisterOverridePostDepsMutators,
 }