As a skilled programmer, do you think it's good investment to learn graphics (Photoshop/Illustrator) or learn a new programming skill (Android)?

That depends on you, are you a programmer that only focused on what called (Back-end development) that its related to functionality and sending results correctly without care how user will see it.
then you will need to focus of skills related to development.
If your work also in Design area or how user will see content of page (Like Front-End Development), you will need to learn graphics design skills, actully for the example mentioned you do need to learn Android too.

If you want to be a specialist don't waste your time learning stuff may be useful maybe not Web Development sth like the big blue ocean you will never cover every thing try to focus in that.

it depends on how you want to complete the circle if you want to work as a freelancer for example you need to now graphics if you want to work as a developer you need learn another language if you need raise your position you need to learn Management courses and so on

It really depends on how you see yourself, do you see yourself as a specialist or a generalist? If you like doing many things but not be specialized in one, then you can of course learn those tools which will help you in graphic design area of web development.
On the other hand, if you'd like to be a professional software engineer, I'd advise not to waste your time learning those tools and rather invest your time in learning new "skills" not necessarily new languages.
New languages can be picked up pretty easily if your are a seasoned programmer.
I am talking about programming paradigms (OOP vs functional vs AOP) or new frameworks like ASP.NET MVC, Web API, etc.

We'll from the industry prospective if you know Graphical Design along with Programming you'll be like a rare coin in the market and you'll get the priority in the recruitment process.
However, if you don't have the (Artist sense and imagination) it's better to go to a new programming skill because Graphical Designing is usually a natural skill.
If you ask a designer how come you can create such talented design? He will reply: Simply I did it.
But in general it is a good to have an idea of doing the basic design operations.

If you an experienced/expert programmer and you only spend time in coding and improving your development skills then you really do not need to go into other stuffs apart from learning android development.
That is a a mobile development usually developed in JAVA for both Android and iOS platforms now its also being done using C#.
So if you are a programmer you should spend your time in learning different languages instead of throwing yourself in learning designing stuffs.
If you are C/C++ programmer then try to learn JAVA.
If you know .NET try to learn PHP.
If you know basic practicing in web development try learning it in Object Oriented Programming if you know OOP try learn MVC.
There are lot to learn being a programmer.
E.g front end programmer, back end programming, JavaScript, HTML/HTML5.
If you have command on all these mentioned technologies then its a very good investment to move towards learning designing technologies.
After all getting more knowledge is a good thing and enhance your career in many ways.

if you are a skilled programmer and has a STRONG graphics imagination, I guess, it is worthwhile to invest in graphics skills, you can apply your programming skills + graphics skill to create a games or animated movies, and as you may know, gaming and animation is a multi-billion dollar industry.
there are so many android developers, and most android applications are almost free, it is nice to have a skill for android, but I would not make it as my sole profession.

Yes, It's very important to improve your skills in programming also learn another skills which allow you to expand your knowledge in various skills which give you the right to be the best employer for a job

I believe it's always better to add more programming skills to your portfolio because using photoshop and illustrator depends on how your sense of design really is which is not a something to gurantee while on the other hand learning a new programming or scripting language will be more useful and much easier because you already have the logic which programming really depends on.

Yes its good investment to learn graphics

I would choose graphics because during my journey with app and software development, designing graphics for my apps was the part that took long time and was challenging. It was the area which make apps stand out in the croud. Most attractive apps have a nice user interface, make the user feel comfortable when he use your app by choosing suitable colours and design professional graphics for you app or software interface.