how to capture any .apk download
Why so difficult? Simply install AppMonster Pro Backup Restore (yes, the Pro version -- the free doesn't do this automatically), and on each install/update the corresponding .apk file is automatically backed up to your SDCard. You can configure the app to keep all .apk files, or only the latest N (number freely configurable), even pinning of a "stable version" to always keep is possible.

 

AppMonster only takes care for the .apk files (not for app data). As a side effect, it even shows you if an app contains an ad module, and you can have it notify you if an just installed app can be moved to SDCard via App2SD.