getFilteredNetworkState is only used in two places, both of which
use only small parts of the NetworkState that is returned to
them. Remove the method and replace it with inline code in the
two callers. NetworkState is a fairly expensive object, and this
removes the need to perform lots of defensive copies of data that
the caller does not need.

Also remove the only call to the NetworkState constructor in
ConnectivityService. That leaves only one caller to the
NetworkState constructor, the one in NetworkAgentInfo. This
constructor is called with defensive copies of NetworkInfo,
LinkProperties, and NetworkCapabilities, so mark these three
parameters as @NonNull. It's also called with a non-null
Network, because NetworkAgentInfo is only constructed with
non-null Networks, so mark the network parameter @NonNull as
well.

In order to make the arguments in the NetworkState constructor
@NonNull, introduce a new constructor that sets everything to
null and make NetworkState.EMPTY call it.

Test: atest FrameworksNetTests
Change-Id: Idcc9e32c53533b0cf61494517e62d4c184fa7610