Unfortunately the main problem is that Android devices allow arbitrary packages to be installed. There will always be someone willing and able to crack and dist

Unfortunately the main problem is that Android devices allow arbitrary packages to be installed. There will always be someone willing and able to crack and distribute any given app that relies solely on client-side security. Your best bet is to roll your own obscure anti-piracy and hope you get most of your sales in the first few days, or look at a solution based on IAP receipts. The same problem exists on iOS, but jailbreaking is an extra inconvenience that only 10% or so of users will accept. Also I doubt many users think twice about downloading a pirated app nowadays (and they should, due to the very real threat of trojans). It's been over a decade since "don't copy that floppy" was replaced by the music/film industry's crusade.