How stops timeline progress

Hey guys, I need help.

I’m working on a project for Gear VR. And in this project I have to have a menu, which when the viewer looks at the button, activates a circular progress. And when it finishes it does the streamload of a new level. That’s okay. What I did, you can see in the pictures I send attached.

My problem is when the viewer looks at the button and stops looking, it would have to stop the whole process of opening the new level. Whenever I look at a button it starts the timeline but not stops when I stop looking at the button.

What can I do to stop the flow when I stop looking at a button and restart when I look at it again?