Don't allow an uncreated Network to satsify requests.

This could happen when another Network changes its capabilities and
updateCapabilities() calls rematchAllNetworksAndRequests() which
calls rematchNetworkAndRequests() on all Networks, even those that
are uncreated.
Allowing uncreated Networks to satisfy requests can lead to bugs
where ConnectivityService instructs netd to perform actions
(e.g. set default Network) on uncreated Networks which netd doesn't
know about yet.

bug:18446301
Change-Id: I857262ac66d1d3af4c264ce128f0a4bee95655de