[IOT-1444] Enabling handling observe requests for collection resource.

[IOT-1444] Enabling handling observe requests for collection resource.

Change-Id: If72f5a85858240787dea2b307710f1167c14c68a
Signed-off-by: Harish Kumara Marappa <h.marappa@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/13149
Reviewed-by: Ashok Babu Channa <ashok.channa@samsung.com>
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Abhishek Pandey <abhi.siso@samsung.com>
Reviewed-by: Jihun Ha <jihun.ha@samsung.com>
Reviewed-by: Hun-je Yeon <hunje.yeon@samsung.com>
Reviewed-by: Uze Choi <uzchoi@samsung.com>