net: lwm2m: get attributes from obj/obj_inst/res for observe_node

1. Read notification attributes set by server to setup the
   minimum/maximum notification period of a observation request.
2. Reordering to check observe_node duplication first
   (bailout earlier)
3. Simplify remove observe_node condition checking

NOTE: attributes are inheritable, priority: res > obj_inst > obj
Reference: LwM2M spec V1_0_1-20170704-A, section 5.1

Signed-off-by: Robert Chou <robert.ch.chou@acer.com>