Convert TestApexPermittedPackagesRules to use test fixtures

Adds PrepareForTestWithNeverallowRules to make it easy to test
neverallow rules. Avoid exporting any unnecessary neverallow related
methods from the android package.

Bug: 181070625
Test: m nothing
Change-Id: Idfc6955cb23f1a4d1790be7879388154b03f3980
diff --git a/android/mutator.go b/android/mutator.go
index 9e99bee..3d24c1e 100644
--- a/android/mutator.go
+++ b/android/mutator.go
@@ -202,7 +202,7 @@
 	RegisterPrebuiltsPostDepsMutators,
 	RegisterVisibilityRuleEnforcer,
 	RegisterLicensesDependencyChecker,
-	RegisterNeverallowMutator,
+	registerNeverallowMutator,
 	RegisterOverridePostDepsMutators,
 }