Increase touch area of SplitView handles for touch-based styles

After QTBUG-82678 was fixed, it's now possible to use containmentMask to increase the touch area for handles without changing their visual size. However, we still need to do this by default for the built-in touch styles (Material, Universal, etc). Ideally we'd rely on the event to know if it's touch or not and then increase the hit area, but short of having that info available to contains(), we'll just have to hard-code it.