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Since 2011, Google has been scanning all apps with Bouncer as they are uploaded to Play and then randomly afterwards. Bouncer does two main things. First, it looks for matches to known malware signatures and, second, it runs the apps in sort of an Android VM to make sure it doesn’t do anything to take advantage of Android in a malicious way. I assume Amazon does something similar, but I don’t know for sure. You can read more about Google security measures on their security blog.