Do not query CS in NetworkProvider constructor

ConnectivityService may not be available in a NetworkProvider
constructor, if it is created (but still unused) before
ConnectivityService starts.

As ConnectivityManager is only necessary in
declareNetworkRequestUnfulfillable, which should not be called often,
just query ConnectivityManager at that point.

This is necessary for VcnManagementService, which is started before
ConnectivityService and creates its NetworkProvider in its constructor.
Fortunately VcnManagementService does not call
declareNetworkRequestUnfulfillable at this point.

ConnectivityManager may be migrated to classic service getters that
cache "null" when the service was not available the first time it is
queried, so no system service must query it before it starts.

Bug: 171540887
Test: atest FrameworksNetTests:NetworkProviderTest
Change-Id: I8dadcd0e1360a9464192f330493e13aa69dd9fe2