net: lwm2m: Observation pmin and pmax refactor

Removed to store pmin and pmax at oberservation node structure and
use attribute list store for calculate time for next Notification.
Observation class use timestamp for triggering notification based on
resource update which use pmin and default pmax behaviour.

Signed-off-by: Juha Heiskanen <juha.heiskanen@nordicsemi.no>