Use a mock PermissionSettings for PMSettingsTest

We were stumbling over ourselves trying to produce a real
PermissionManagerService, just to get a real PermissionSettings instance
that is irrelevant to the test. This change mocks the instance which
makes for a more isolated and faster test.

Fixes: 140267376
Test: atest PackageManagerSettingsTests
Change-Id: I0513fca23de410bb33d79ae865787472f0aaec23