Bluetooth: Mesh: Support for storing heartbeat publication persistently

Add support for storing the heartbeat publication persistently. The
information is only stored as "publish indefinitely" or as "periodic
publishing disabled" since we can't know for how long the node is
powered off. The information is stored under the settings key
bt/mesh/HBPub.

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