Delta calc: Only hide the +/- buttons when typing an equation

This addresses a comment left in a [review](https://play.google.com/store/apps/details?id=com.nolanlawson.keepscore&reviewId=Z3A6QU9xcFRPSGRGb05FVXI3SHlMYlJQVVhReEJhTXlSRzl0SUZEUEtjS0YxeTljT1JnWE0zckdKVDlUdEs2T1NFWVBfaktpcXVfblUwRUZjMlFLMEV2dGc&hl=en).

>  The "pop-up buttons" shouldn't disappear after you press one. You should be able to keep hitting them until you get the value you want. It's a pain only getting to hit them once each time.

The increment buttons are only hidden after the user types one of `+-*/` into the text box.