Bluetooth: Mesh: Fix checking for active heartbeat publication

When sending heartbeat messages triggered by feature changes the code
was trying to look up the configured publication subnet, in an
apparent attempt to figure out if publication is enabled or not. A
more appropriate way is to check for the heartbeat publication
destination address, and since we have a helper function this can be
done in a single place.

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