Add android support

[sienna-android](https://github.com/crumblingstatue/sienna-android) doesn't break the game on PC, [it only adds android specific code](https://github.com/crumblingstatue/sienna-android/compare/SimonLarsen:master...touch), so there is no reason to have two different codebases.

Others can still contribute Android-specific features by submitting pull requests to this project.