"A malicious Android app that downloads itself from advertisements posted on forums "
Try again. I does nothing of the sort. even withstanding you need to have turned off only allowing browsing in the Google Store, after ignoring the warnings of doing so, AND you oped out of the app scanning, even then, it doesn't "download itself". It's a message dialog generated by the BROWSER that is trying to fool you into downloading and installing an APK.
If you can't understand these basics, should you really be writing about security?? Just sayin'