[VCN03] Linger the newly connected network for a short time

This is mainly used by VCN, where VCN-eligible networks might
be registered before VCN management service can fire request
to prevent the newly created network from being torndown
immediately by ConnectivityService.

Test: atest ConnectivityServiceTest#testNewNetworkLingering
      atest ConnectivityServiceTest#testNewNetworkLingering_bgNetwork
      atest FrameworksNetTests --rerun-until-failure 20
Bug: 175180558
Change-Id: I7768daf4cf2d8f6d8057b5107412e06104ba82e0