Modify subscriber list in Invite resource

Modify subscriber list in Invite resource

- modify Invite resource to make multiple devices(for each user)
  can get observe notification about invitation info.

Change-Id: I4afa8cf0e4da5c68c3478cb7903afd3bf14dc97b
Signed-off-by: Minji Park <minjii.park@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/12001
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: jung seungho <shonest.jung@samsung.com>
Reviewed-by: Jee Hyeok Kim <jihyeok13.kim@samsung.com>
(cherry picked from commit 558d3f1b6287f139ec75361b3e0cdfdebec38b08)
Reviewed-on: https://gerrit.iotivity.org/gerrit/12113