Support Safemode for VCNs.

This CL updates VcnManagementService to support Safemode for VCN
instances. Specifically, VcnGatewayConnections will notify their Vcn
instance when they enter Safemode. Vcn instances will in-turn notify
VcnManagementService, which notifies all registered
UnderlyingNetworkPolicyListeners to update their policies.

Bug: 178140973
Test: atest FrameworksVcnTests
Change-Id: I3336c150e9406b3eb2330d2e86cae2ed835730bb