How can I have EV3 Commander App continue running the motor when I release the slider?
The commander app is very limited in it's capabilities. Unfortunately it is not possible to control your own program with the commander app. 

I have tried all of the built-in EV3 Commander control pads to see if any of them were suitable. The closest thing I could find to a solution would be to use the 'EV3RSTORM' control pad. 

The 'Go', 'Back', 'Left' and 'Right' buttons keep all the motors turned on permanently unless you press stop of change directions. Unfortunately you won't be able to change the variable speed of the motors but you could change the fixed speed with gears.