Honor the XKB_FIXED_LAYOUT/XKB_FIXED_VARIANT udev properties

https://bugzilla.gnome.org/show_bug.cgi?id=775681

Continuing this issue because I want to implement XKB_FIXED_MODEL (https://github.com/systemd/systemd/pull/13896) and the other two XKB_FIXED_* properties as well.

There is two problems I currently don't have a good solution for

1. If the keymap is changed based on the device the event is coming from on a multi seat setup it's constantly changing when both devices are being used.
2. there doesn't seem to be an easy way to report a changed keymap from eithe MetaDeviceManagerNative nor MetaSeatNative.

Any ideas how to overcome those issues?