Fixes https://github.com/flutter/flutter/issues/142045

The intent of using `??=` was that if the tooltip is already scheduled for showing, rescheduling another show does nothing. But if the tooltip is already scheduled for dismissing, the `??=` won't cancel the dismiss timer and as a result the tooltip won't show. So the `??=` is now replaced by `=` to keep it consistent with the `_scheduleDismissTooltip` implementation.