common: Ignore (pure) keyboard devices on GsdDeviceManager

They are not needed for GsdDeviceMapper purposes, and can cause misbehavior
if the keyboard device "shares" the input node with a device we do need
mapping.

Fixes spotty touchscreen detection with those touchscreens that have a
capacitive menu/windows button.

https://bugzilla.gnome.org/show_bug.cgi?id=787884