Whatever you do to secure your secret keys is not going to be a real solution. If developer can decompile the application there is no way to secure the key, hiding the key is just security by obscurity and so is code obfuscation. Problem with securing a secret key is that in order to secure it you have to use another key and that key needs to also be secured. Think of a key hidden in a box that is locked with a key. You place a box inside a room and lock the room. You are left with another key to secure. And that key is still going to be hardcoded inside your application.
So unless the user enters a PIN or a phrase there is no way to hide the key. But to do that you would have to have a scheme for managing PINs happening out of band, which means through a different channel. Certainly not practical for securing keys for services like Google APIs.
