Hi Zeev,
you may consider to build your own solution in order to manage licensing. I'm thinking aloud here, so take it as something that needs a deeper analysis and absolutely not a guidance or hint.
To use your app, a customer needs to register with you (and possibly pay..). For registration you use his/her email and a userID that you generate on your server and mail him/her. Upon installation of your app, the customer needs to enter that userID, but the app will generate a device ID too (a random number, or any other key you like), then send it to your server along with the user ID (everything encrypted, just to make it more solid). Only upon successful registration the app starts working.
From that point on, on startup the app will send the couple user-device IDs to your server in order to properly work.
Now, on your server you have full control about the licensing since you know email, userID and deviceID, so you could switch it off at any time.
Let's say that your legitimate user buys a new phone. You app gets installed anew, the userID stays the same (since the customer knows it) but the deviceID will be different.
When the registration message reaches your server you can disable previous account and start the new one. This prevents the user to give away his/her userID to friends or others since his/her own account will stop in that case.
What about a customer that wants to use the same app on two phones? Just mail him/her two or more userIDs.. after all, all the ID for the same customer will share the same email on your DB so manage them will be easy for you.
What about people who investigate about the deviceID and are so able to generate a twin of a legitimate installation? Well, it depends on the nature of the app. If it works with personal data I doubt that the original user will be willing to let it happen. If it's not, are you sure taht the economic damage you suffer from a few pirated copies justifies a great amount of time to build a more secure solution?
Again, all the above needs deeper analysis but it could be a starting point.
