Bluetooth: Mesh: Prov confirm tx after confirm rx

According to Mesh Profile Specification 1.0.1 Figure 5.17, the
unprovisioned device should send its confirmation value after the
provisioner sends theirs. Previously, the confirmation value would be
sent immediately after OOB input complete. Now it first waits for the
input data, then from confirmation from the provisioner before sending
the confirmation.

Fixes: #18178.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>