Bluetooth: Mesh: Initialize PB-GATT advertising data at the right time

Some things, such as UUID, URI or even the local name may be different
when bt_mesh_prov_enable() is called compared to when bt_mesh_init()
was called. Create the advertising data on-demand each time when
enabling PB-GATT.

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