Is it possible to export to a standalone .apk file for android?

You saw the answer about AIR 3, but in the meantime you perhaps shouldn't worry so much about the issue. The messaging that the user sees isn't too terrible, and only the first AIR app the user installs would show the message. If your app gets installed onto a device that has an AIR app already, then the user won't see the extra message.

Well it is possible for testing because that is a public beta, but you will have an "Air 3 beta" water mark until its officially released.

I encourage you to try it. I have recently packaged my app with Air 3 with captive runtime on android. I am finding many new performance issues...

crashing/lagging/graphics disappearing. I mentioned it an other threads, no one is commenting on it here but i cant imagine its just me. Please try Air 3 on android (via that link to the SDK) and report your experiences.

No, this is not possible - not yet.

With AIR 3.0 this feature will be implemented:

Captive Runtime Support (AIR for desktop and Android) — This popular feature from AIR for iOS is now available for the desktop and Android device. Developers now have more control over the use experience with this new packaging option. Simplify the app installation process by packaging the AIR runtime with your app on Windows, Mac, and Android. Developers can now control exactly which version of the runtime an app executes against. Deploy AIR application in tamper proof /hardened operating environments (e.g. medical diagnostic equipment, government, military).

Have a look at the information for the 3.0 beta:

http://labs.adobe.com/technologies/flashplatformruntimes/air3/