Correct ExpandedMobileWorkerFilter behavior when users are specified

Addresses this ticket:
http://manage.dimagi.com/default.asp?115291#734337

Without this change, passing a user to the EMWF will show all cases owned by this user or owned by case sharing groups of which this user is a member. This change makes it so that only cases owned by this user will be shown.
Yes, but not until after the `group_owner_ids` list has been populated. So, `group_owner_ids` will only be populated with the groups that contain users that were contained in a group passed to the filter. Users that were passed by name to the filter will not cause groups to be added to `group_owner_ids`.