Bluetooth: Mesh: Move storing of new prov data to settings work item

This allows to move flash operations from the context that triggered
bt_mesh_provision or bt_mesh_reprovision to the mesh settings work item
that runs on the system workqueue. This is required to for a case where
the mesh settings work item is running on a separate thread instead of
the system workqueue to unblock the system workqueue eventually.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>