That's a good question, I think this might be possible with one of the control options that uses buttons, but I'd have to test it to be sure. It's been a bit since I've played on mobile.
With Android, you can't do the right click hold trick. However, to make up for that, you can quickly harvest and water with tapping quickly over the crops. Just be careful to make sure only the scythe or watering can are equipped or else ya might accidentally pickaxe a crop to an early demise when doing the quick taps method.