Code obfuscation or License library ?

Why not use both of these methods?

Could be an idea!
My doubt is about Licence Library or better about Licence management.

Licence Library questions :

1) Is licence management compatible on all Android versions?
2) How much is the effort to implement it ?
3) The added value compared to code obfuscation is significant ?

You should obfuscate your released code.

Code obfuscation makes it more difficult to reverse engineer your code. Anyone with a rooted device can still take your APK file and distribute it. Licensing makes it more difficult to run your APK on other devices. You should see the Licensing tutorial for more information.