1. Make addLinkAddress a no-op if the address already exists.
2. Make addLinkAddress, addStackedLink and removeStackedLink
return a boolean indicating whether something changed.
3. Add a removeLinkAddress method (currently there is no way of
removing an address).
3. Move hasIPv6Address from ConnectivityService to
LinkProperties, where it belongs.
Bug: 9625448
Bug: 10232006
Change-Id: If641d0198432a7a505e358c059171f25bc9f13d5
4faa027df0