Add UpstreamNetworkState for tethering

Previous tethering use NetworkState to encapsulate network states.
But NetworkState is hide API and Tethering just use NetworkState
locally that it don't interact with other module by NetworkState.
To avoid hide API accessing and reduce dependency, replace
NetworkState as UpstreamNetworkState in tethering.

Bug: 144758082
Test: atest TetheringTests
     OFF/ON hotspot

Change-Id: Ia1837b6ed983a64c3ed72ecf0e6f46d91b3af7da