Handle empty permissions for packaged apps (bug 1039067)

https://bugzilla.mozilla.org/show_bug.cgi?id=1039067

Handle the case where there aren't any permissions in the manifest which the app-validator helpfully represents as `False` instead of `[]`.
When the app doesn't define a `permissions` key in its manifest the app-validator sets `"permissions": false` in its validation results. So I moved the default outside of the `get`. I meant to add a comment. I'll do that. 

I'll switch back to the `any`. I had just noticed that `PRERELEASE_PERMISSIONS` was already a set so I did intersection.