Optimize sizing

While I'm aware of !340 which will solve the same problems, and others as well, it seems it's still far from ready. Which is why I believe it still makes sense to implement a simple, easy-to-revert, fix for one of the most obvious and painful use-case: using on a device other than the Librem 5 or PinePhone.

This MR makes sure the current behavior is preserved on these phones, with a slight change to improve usability in landscape mode. But at the same time, it introduces pretty straightforward changes in the way the keyboard height is calculated so that squeekboard can be used on devices with different display resolution/density, such as the PineTab.