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