This has to be the best free Android app on the market for Koi-Koi. It has been updated a few times since it was launched (as for instance at one point it erroneously included the 'Rain-man' for the Yaku of Sanko and Shiko).
There are six different 'Samurai' to defeat and although I'm a veteran Koi-Koi player it did seem a little too easy to clear all of the stages. Still, it is free and good enough for a bit of Koi-Koi on the go!
Give it a go!