net: lwm2m: get attributes from obj/obj_inst/res for observe_node
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 <563fc56b80720efb29939356d471536b8c17d514@acer.com>

zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,explora26/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,finikorg/zephyr,zephyriot/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,punitvara/zephyr,galak/zephyr,ldts/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,explora26/zephyr,ldts/zephyr,finikorg/zephyr,ldts/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,punitvara/zephyr,nashif/zephyr,punitvara/zephyr,mbolivar/zephyr,zephyriot/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,finikorg/zephyr,mbolivar/zephyr,punitvara/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,kraj/zephyr,mbolivar/zephyr,zephyriot/zephyr,zephyriot/zephyr,kraj/zephyr,nashif/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,nashif/zephyr