Well considering this is the kind of stuff Google are selling on their Play store LINK
I hate to think of the stuff you can just download from anywhere. Yes, a great idea.
Whilst Microsoft Windows and Android are the most popular areas for Malware as they are by far the most widely used operating systems I dont think you will find anyone is immune:
Just like on your PC, Mac, iPhone etc caution should be taken when installing any app.
The Amazon app store (i believe) claims to be running more security controls on its apps if you choose
to use a diferent store