header-bar: Refine mobile window detection

This widens the mobile screen width to 480px and don't check the height
for vertical mobile detection to be a bit more inclusive, and at the
same time this considers only maximized windows as mobile to avoid
replacing the close button by a back button to close a floating window
on the desktop.

Fixes https://gitlab.gnome.org/GNOME/libhandy/-/issues/315