For the time being, PolyNome is iOS only.
It took 5+ years of full time work to get it to the stage it's currently at, and I'm still not finished!
As much as I'd like to have it available for Android, it won't be happening in the near future.
In the meantime, many people have bought second hand iOS devices purely to run PolyNome. You can pick up a second hand iPhone or iPod touch that runs iOS10 for less than the cost of a lot of physical metronomes that do far less.