As far as I know, I was at a security conference in January, there is no code signing and no sandbox on Android. That is not a good scenario for "security".
Code signing is involved, to verify that particular apps have the same source, and that updates have the same source as the original app. (That's about all it's good for.)
Briefly: each signing key gets its own UID on the phone. Unix file permissions are used to restrict access to files, devices, and other facilities. In some cases, kernel hacks are required to make this possible; the Android kernel only lets you open a TCP socket if you're a member of a particular group.
If there's a local root exploit, you can still bypass all of this --- but the same is true on iOS.
