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Try out RD4. It's got a mono synth, 303 bass like synth with sequencer and a drum machine. You get four slots to add any of the three instruments. Or out in of the three in all slots I you want.
It has midi support.
It works well with SPC, which is a pad looper sequencer sampler thing. It's pretty dope.
Haven't used it myself with a keyboard but you can mess with the latency settings and whatnot to make it work well, I hear
Edit: rd4 demo link https://play.google.com/store/apps/details?id=com.mikrosonic.RD3XDemo
SPC demo link: https://play.google.com/store/apps/details?id=com.mikrosonic.SPCDemo