Bluetooth: Mesh: Introduce a second advertising callback

Some places of the code are interested in when the first advertising
event goes out. Others, on the other hand, are interested when the
last advertising event goes out. Some are even interested in both of
these. Instead of providing a single callback, provide a struct with
two possible callbacks for sending advertising PDUs.

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