It seems like there is no official Android app, and I’d very much like to see one.
I understand there are 3rd party apps but I would feel more comfortable in buying a lifetime subscription if there was an official app, as 3rd party ones seem to be quite unreliable (a good example is https://play.google.com/store/apps/details?id=tr.xip.wanikani).
They were working on an official iOS app earlier in the year (to be followed by an Android app), but early testing didn’t go well. I think they decided to go back to the drawing board, and there hasn’t been an update in a while.
I third Flaming Durtles (if you can second you should be able to third too IMHO)
The only downside (well depends on what you consider a downside) is that it takes a while to get used to all the ways you can set it. Just explore the settings and see what kind of things it can do, it can be a bit confusing at first.