[KeyTimer] Time for countdown is retained after countdown has finished

### Affected hardware version

Bangle 2

### Your firmware version

2v23

### The bug

After setting a countdown with the KeyTimer app and waiting for it to finish, when I open the app the next time, it still has the time I had set last. I consider that a bug - or at least potentially intended, but unwanted behavior - because the point of a timer app like this is to be able to quickly and easy set a countdown - which is noticably delayed by always having to clear out the old one that is almost always not the one you want to set in that moment. Also, since the buttons are quite small and fingers sometimes aren't, every button press that can be avoided should be avoided. So:

Proposed change:
Make it so that when launching the app, the timer is reset to 0 so you can set it from scratch. Failing that, make it so that the timer variable is reset when the countdown ends or is aborted - this way, the typed value is retained when merely closing and re-opening the app, but in any scenario where you'd want to set a new countdown, it's already reset.

### Installed apps

_No response_