[FUI22] Support getAllNetworkStateSnapshot

Currently, ConnectivityService has getAllNetworkState but it is
not ideal to expose as system API since the plan is to get rid
of NetworkState. Thus, create a new one that returns
NetworkStateSnapshot to fulfill the needs.

Note the original getAllNetworkState cannot be deleted now since
it has @UnsupportedAppUsage annotation.

Test: atest FrameworksNetTests
Bug: 174123988
Change-Id: Icddd434552b0e9ecbc8299e7242ec88cf3145aca