Rename ADDON_ICON_SIZES and only generate the ones we need (bug 885329)

https://bugzilla.mozilla.org/show_bug.cgi?id=885329

We should probably deprecate 32x32 and 48x48 as well, but that's for another bug.
this test should be using a patch (or something) to set a value for APP_ICON_SIZES so it always has a guaranteed value.