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.
Made this behavior optional to preserve behavior of `DailyFormStatsReport`.