[IOT-1671] Fixed base layer terminate logic

[IOT-1671] Fixed base layer terminate logic

- ObserverList should be removed before destroying ResourceList
to avoid the error of dangling pointer. because ResourceObserver
object includes a resource handle which is memory address allocated
when resource is created.
- Add function to unset network monitoring callback when OCStop()
is called.

bug: https://jira.iotivity.org/browse/IOT-1671
Change-Id: I8a367a24a2100c97bf612db26bc41db5f47f2feb
Signed-off-by: hyuna0213.jo <hyuna0213.jo@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/15499
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>
Reviewed-by: jihwan seo <jihwan.seo@samsung.com>
Reviewed-by: Ashok Babu Channa <ashok.channa@samsung.com>