Google will soon start adding security metadata to APKs distributed by Google Play to identify their origin. The change is set to primarily affect users in areas where mobile data is at a premium, either due to price or availability, which leads to APKs being frequently shared peer-to-peer.
Plans for this change were announced late last year. The authenticity of "apps obtained through Play-approved distribution channels" will be able to be determined while devices are offline, and the apps will be added to their users' Play libraries. They'll then receive updates through the Play Store when possible.
Developers won't have to do anything differently; all these changes will take place on Google's end. Google says that developers will actually benefit from the change, as apps shared peer-to-peer in areas with poor connectivity can now be kept up to date by the Play Store.
- Android Developers Blog