compositor: set data_device keyboard focus in seat_get_keyboard

compositor: set data_device keyboard focus in seat_get_keyboard

Because seat_get_keyboard is called after initial
weston_surface_activate, wl_data_device_set_keyboard_focus
fails to send data offer for newly connected client due to
wl_seat.focus_resource being NULL.

This patch calls wl_data_device_set_keyboard_focus
in seat_get_keyboard, so it can send data offer for
newly created client (when wl_keyboard.resource_list
and wl_seat.focus_resource are properly set up).

https://bugs.freedesktop.org/show_bug.cgi?id=60617