samples: boards: nrf: system_off: use PRE_KERNEL_1

Sample was using PRE_KERNEL_2, but the action (lock OFF state) can be
run at any time before the Kernel starts, ie, PRE_KERNEL_1/2. Use
PRE_KERNEL_1 + 99 to preserve similar behavior.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>