Custom keyboard shortcuts for Android
Not every device has this feature. I tested on my samsung-galaxy-s-3, and it doesn't have this option either, but the nexus-7 and nexus-10 do. I'm not sure whether the feature was only added in 4.2-jelly-bean, or if it's just missing from Samsung ROMs. Either way, you probably want a third-party text expander app as others have suggested.