Bluetooth: Mesh: Fix heartbeat subscription state handling

PTS version 7.3.0 incorporates some errata which change the expected
behavior of the heartbeat subscription state. Update the code so that
the following tests pass successfully:

MESH/NODE/CFG/HBS/BV-01
MESH/NODE/CFG/HBS/BV-02
MESH/NODE/CFG/HBS/BV-03
MESH/NODE/CFG/HBS/BV-04

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