Stop using NetworkState internally in ConnectivityService.

NetworkState is used by many methods that take or return legacy
network types. It is used because it contains most of the state
related to a network.

This code is not easy to follow and is more expensive than it
needs to be: most of the methods that construct a NetworkState
only really need one of its members (e.g., LinkProperties, or
NetworkInfo), but constructing a NetworkState requires making
defensive copies of all its other members as well.

- Instead of using NetworkState, use NetworkAgentInfo, which
  already contains all the same data.
- Replace calls to getUnfilteredActiveNetworkState with calls to
  getNetworkAgentInfoForUid. When getUnfilteredActiveNetworkState
  returned NetworkState.EMPTY, return a null nai, which causes
  any caller to see return null LinkProperties/NetworkInfo/etc.
- Rename filterNetworkStateForUid to getFilteredNetworkInfo,
  because that's the only thing it actually filters.
- Rename getFilteredNetworkInfo to getFilteredNetworkInfoForType
  to avoid having two methods with the same name perform two
  different operations. That method was only introduced recently
  in aosp/1552503, so it's probably fine to rename it.

Bug: 174123988
Test: passes existing ConnectivityServiceTest
Change-Id: Idfb5e149967266a442b268de6f13a521884dbb8f