Hi Justin,
Regarding the CMS.WebAnalytics.ActivityLogHelper.UpdateContactLastLogon method - this method is a handler, which is automatically fired once an activity is logged. It was made private in version 8. Is there any specific scenario when you need to call this method and you are not logging an activity at the same time?
As for the CMS.WebAnalytics.ActivityLogProvider.LogLoginActivity method - in version 8, you can use the CMS.WebAnalytics.ActivityLogProvider.LogActivity method instead and supply the parameter's (ActivityData) Type property filled with CMS.WebAnalytics.PredefinedActivityType.USER_LOGIN.