[IOT-1615] Correct return type of callback in SubOwnerClient

[IOT-1615] Correct return type of callback in SubOwnerClient

Sample only built when MULTIPLE_OWNER=1 has callback that differs in
return type from what's required. Change return type to the required
OCStackApplicationResult and have the callback return OC_STACK_OK.

Bug: https://jira.iotivity.org/browse/IOT-1615
Change-Id: I5f787f64a16f308c182067d20009decd29760730
Signed-off-by: Kevin Kane <kkane@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/15109
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Randeep Singh <randeep.s@samsung.com>
Reviewed-by: Alex Kelley <alexke@microsoft.com>
Reviewed-by: Greg Zaverucha <gregz@microsoft.com>