Fix incorrect state of add/remove actions for some favorites.

KService::serviceByStorageId is quite lenient (which is nice), but
means we need to update to the storageid of what it ends up finding
instead of sticking to the one we started with, or the comparison
against the service instance backing the favorite can fail later.

Concretely, this currently happens when foo.desktop is turned into
kde4-foo.desktop for the unported apps in the default favorites.