Fix FindGroup API of Things Manager about group sync feature.

Fix FindGroup API of Things Manager about group sync feature.

It is not notify about state of finding to caller.
So we will return 'OC_STACK_ERROR' when It was finding a group.
User should be wait findCallback.
Then user will know the result of finding.

FindCallback has the argument which OCResource.
If it is NULL we could not find a group.

Fix [IOT-286]

Change-Id: I2b95d5102e54e4242394c36d51de81f192b87e13
Signed-off-by: HyunJun Kim <hyunjun2.kim@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/230
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Uze Choi <uzchoi@samsung.com>