Force full screen refresh with a rounder function

When the screen switches, the full screen needs to be refreshed for the
hardware scrolling to work. This was enforced with backgroundLabels, but
is simpler to do with a rounder function.

src/displayapp/screens/Timer.h

@@ -24,7 +24,6 @@ namespace Pinetime::Applications::Screens {
     uint8_t secondsToSet = 0;
     uint8_t minutesToSet = 0;
     Controllers::TimerController& timerController;
-    lv_obj_t* backgroundLabel;
     lv_obj_t* time;
     lv_obj_t* msecTime;
     lv_obj_t* btnPlayPause;