Change default latency policy to "Force smoothest animations"

There are some performance differences between X11 and Wayland. Desktop
systems are mostly unaffected by them, but laptops suffer a bit.

On Wayland, kwin always does double buffering. This is great for
reducing latency and avoiding tearing, but if the gpu can't keep up with
the work, you're going to see stuttering.

Another issue is that in order to reduce latency, we need to have very
good frame stats. At the moment, kwin records only cpu render time, but
we also need to record the gpu time. We've already done some work in
this area, but it's most likely Plasma 6 material. (plasma/kwin!1163)

In the meantime, let's change the default latency policy to "Force
smoother animations." It's going to improve frame rate. If people care
about latency, they can change latency policy in system settings; the
option is still there.

CCBUG: 452119