Activation key

Hi all, I have a free app and an app that works like an activation key, so that if you installs App 1 you have it in a demo version, with limitations and ads, then if you buy and installs App 2 you unlock full features in App 1. To achieve this I put a check at App 1 start to see if App 2 is installed (through activity starter). This is working fine, however I have some doubt.

1) I'm afraid that one can simply backup App 2 and pass it to others
2) I don't like this method so much, because you have to keep App 2 installed

So I was wandering how will you do something like this.
I'm thinking about using tinywebdb to make user login and save its credentials along with an unique code and when App 1 is started check if user credentials exists. But then others issues come up, like: how to delete credentials when user unistalls the App 2? Because If it is a pro-user he will simply install App 2, activate App 1 and then unistall App 2 getting the refound by google market, and so he will get full unlocked app without give me a penny for my efforts.
Any thoughts and considerations will be much appreciated.



to avoid the refund problem, App1 could check if App2 is installed as currently

if yes, then store this information and the current timestamp in TinyDB. On next run of App1 check if more time than the refund period has passed and App2 is still installed. If yes, display a message for the user "you can uninstall App2 now")