Now get another access token if saved is invalid & encryption key different for each computer

I have include an option to be able to get another access token when the
saved one is invalid. It is necessary to add the constant
MAX_TRY_GET_VALID_ACCESS_TOKEN in Constants.java to be able to control
that. If you don't wan't to ask for another token you just need to fix
it to 0 otherwise you can set it a anything. For example I have set it
to 3 like this
public static final int MAX_TRY_GET_VALID_ACCESS_TOKEN = 3;
in Constants.java
Added a small change in ecnryption process. Sure it's great to encrypt
but I think it's better if  key is different for each computer. Here we
have a chance on 2^127 to get the same key. And not every one is
changing of mac adress every day