Since the release of the Adobe AIR 3.8 SDK, all Android apps are packaged as captive runtime, regardless of the packaging options you select in IDEs like Flash Builder.
To override this behaviour, simply add the following environment variable to your system settings:
Restart your IDE and the next time you package your Adobe AIR app for Android, you’ll be able to use the shared runtime as before!*
* Mixed results reported by users of Flash CC, works every time with pretty much everything else