Add \OC\App\Manager to handle enabling/disabling apps

Add a proper public interface for enabling/disabling apps.

I would like to expend this in the future and refactor all off `OC_App` in a proper public interface later.

cc @DeepDiver1975 @PVince81 
This will return false when not logged in and the app is enabled for specific groups only, in public.php you'll want to call `isInstalled` instead (see https://github.com/owncloud/core/pull/9962/files#diff-47860299e627b9b2b9246d36f1dc6f2aR40)