Android, at least those with Play Service, already have built-in scanning for APKs right now. The main problem is currently only Google can "trust" the dev signature for apps they release, so if an app or dev is in Google's naughty list, either for outright malware or for an adblocker, they're out.

At least there will be a setting for users to allow any app from any dev, which those who know about it will enable right away, and those who don't know about it have no business to use it anyway.