added localization for strings + sample code for demonstration

[Atternatt]

I've added generic localization in core and n android implementations (with sample code).

[franmontiel]

I propose to make the methods return String instead of String? In my opinion if the string is not found is an error and also is much better to handle a non-nullable value.

We can change it to throw an exception, maybe IllegalArgumentException("key do not belong to any string"), if the String is not found.

We already know that in Android we will always find it unless we pass a random integer and this deffinitely should throw and Exception.

[Atternatt]

indeed, I'm agree @franmontiel I've just redone it propagating the same exception that the sdk is throwing when it doesn't find a string throw Resources.NotFoundException()