Use 'app_id' consistently in form drilldown filter

fixes http://manage.dimagi.com/default.asp?146640
followup from https://github.com/dimagi/commcare-hq/pull/4900
@nickpell FYI, but anyone is welcome to merge :P
this is just reformatting, except I changed `'app'` to `self.app_slug` (which is `'app_id'`).