distinguish between iarc-excluded and developer-excluded AERs

Noticed that if a developer got a rating, then decided to exclude Germany + Brazil, then updated their rating, their exclusions would get wiped out. So this patch distinguishes between when we exclude an app from a region due to IARC, and when we excluded an app from a region because the developer checked it so.

I went with a simple flag, not very extensible in case we ever use AddonExcludedRegion for more purposes, but this'll do short-term.