Implement missing WARM behavior for Thread router devices

[jaylogue]

The WARM support code in the OpenWeave Device Layer is incomplete with respect to certain operations needed by nodes operating as Thread routers. In particular, the StartStopThreadAdvertisement() function (in WarmSupport.cpp) is unimplemented. This results in an inability to communicate using Weave ULAs when all the members of the network are Device Layer-based Thread routers.