modules: openthread: configure MLE and supervision in openthread_init

PR 84355 introduced configuring the child supervision and
the MLE timeout in openthread_start() (later replaced with
openthread_run()), but that function is only called when
the manual start is not used.

Nothing prevents configuring these settings earlier, in
openthread_init(), which is executed even if OT autostart
is disabled.

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>