There isn't really one. You can play using Steam Connect, but obviously that requires a PC first (the saves are transferable between PC and Android). I fought hard to keep it under 2GB, including setting up a custom server to download additional content, but paying for hosting for downloading 2.5 (or 8GB+ for book 3) additional data for everyone who downloads the game on Android, while not really making any money of it, is obviously not a great business decision.
Since it also required a completely different way of uploading and building the game, I eventually stopped when just the base application + videos went over the 2GB threshold.