Fix initial regions paid apps (bug 1055513)

r? @robhudson 
Calling `get_region_ids` should return the regions this app is currently listed in. By definition that is all regions minus the excluded regions, taking into account the IARC excluded regions and paid app excluded regions. That's what `get_excluded_region_ids` does, correct?

The list of addon_excluded_regions is the ones that the user has chosen where as `get_excluded_region_ids` is the ones the user has chosen plus the ones the system has overridden for other reasons.

At least that's the way I understand it.