@arminkh everything is hackable/crackable, nothing is safe and there's absolutely nothing you or I can do about it. Listen if Facebook, Twitter, Apple, Adobe, the Pentagon and NASA can all get hacked into just to name a few, then there absolutely nothing you, I, anybody on the forum and yes that even includes
@Erel can do to stop it from happening. So I suggest that you do the best that you can to protect your app and get on with your Android project.
Basically what I'm saying is that if a half decent hacker/cracker really want to get in to see the basics of your Android apps source code, then nothing will stop them. I've learn from from the fact that my best selling Windows program has both been cracked(no unlock code needed) and there's also a keygen for it out there too. Changing the formula is a waste of time as all you have to do is request that is get cracked again and within a month it is done and a new keygen is released. I gave up attempting to stay one step ahead of keygen creators years ago. I now just concentrate on making my products better for my customers...