Bluetooth: Mesh: Fix publication period timestamp initialization

The period_start timestamp wasn't getting properly initialized in the
case that retransmission was not being used. In the case of
retransmission the timestamp was getting updated in the mod_publish()
delayed work callback. Add a send_start callback and do the
initialization there, since this covers both the retransmission as
well as the no-retransmission cases.

Fixes #17820

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>