As noted in our VIdyo conversation:

* I'd prefer the inputs and outputs included the binary blobs instead of using S3 are part of the queue
* However this works, as long as we don't break the existing API with the client (the APK Factory streams the signed APK file back as the response)
* The APK Factory service manages a cache of APKs, I'd prefer to keep that knowledge in one place, but I'll go with your call on depending on APK Signer writing the signed APK to the final S3 location and keeping the file around for a long time