Shrink the surface to scan. Not foolproof but can be an improvement. After watching many conference videos on mobile OS hacking, the strategy should really be to focus on components that were touched recently in new updates (and the most popular components like messaging). Once software is mature, they don't go and change everything with every release. It's been a few years, but an interesting insight I learned at CCC was that the most touched components in new OS releases are the ones most likely to have contributions by junior developers trying to push some commit to satisfy their boss and thus have security flaws. Testing can only catch so much.