backends/native: Pass xkb_state directly when updating MetaKeymapNative
Figuring out the MetaSeatImpl this much indirectly is fairly awkward when the keymap is only updated from the MetaSeatImpl, pass instead the seat impl's xkb_state, as we have it handy in all the places this is called.
This will not break on NULL seats during initialization, should the numlock state be restored from previous sessions.
Fixes: #1556 (closed)