Android : Protect android source code

Hello!

I am writing android application. Now I want to protect Activity classes. I use proGuard for obfuscation but when i decompile my apk with fernflower i can see full source code. I downloaded apk from play google and tryed to decompile it and could not decompiled onCreate method. Maybe someone know how could i protect my code. Maybe custom class loader? But how i know it's need to pass the path to dex file but if apk will be odexed my programm will fall.