Bluetooth: Mesh: Fix setting health period divider

A previous patch which moved dispatching the health publish callback
to a later moment introduced a regression where the period divider
does not get updated when it should. In fact, having the divider as
part of the Health Server context is redundant, since the same
information is already stored generically in the model publication
context. Switching to using the model publication context makes things
simpler and ensures that the value is always up-to-date.

With this patch it is possible to pass MESH/SR/HM/CFS/BV-02-C.

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