Bluetooth: Mesh: Fix heartbeat sending on Friendship established/lost

Mesh Profile 3.6.7.2 Publishing Heartbeat messages:

"
Triggered publishing of Heartbeat messages is enabled by
the Heartbeat Publication Features state (see Section 4.2.17.5):
...
- If the Low Power bit is set to 1, a Heartbeat message shall be
 published when the node establishes or loses Friendship (see Section
 3.6.6.1).
"

Fixes #18194

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