Google occasionally updates its Play Store rules to weed out inconsistent or shadybehavior in Android apps, and we're witnessing one of its larger clean-ups today. The company's new guidelines more explicitly ban device interference: titles in the Play Store can't modify settings or other apps without permission, and they can't install bookmarks or icons that pitch a third-party service. Google is also adamant that any in-game purchases of virtual goods must go through its billing system. As usual, new apps have to follow these guidelines right away, while developers with existing apps have a 30-day grace period to make any changes. The policies won't necessarily stoprogue code from sneaking into the Play Store, but Google can at least say that it gave fair warning.