Move OCGetServerInstanceId in OCStack

Move OCGetServerInstanceId in OCStack

Server instance id is needed by Resource Directory as it is a required field.

Currently getting Device Id is present in ocstackinternal.h but is needed to be accessible across.

This does not change any code but just moves API from ocstackinternal to ocstack.

Change-Id: I4f424a0fa498f3793933cd94294d9d8f991d4258
Signed-off-by: Habib Virji <habib.virji@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/2635
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Madan Lanka <lanka.madan@samsung.com>
Reviewed-by: Patrick Lankswert <patrick.lankswert@intel.com>