Allow soft-start and opportunistic safe mode

This change adds support for opportunistic safe mode, where the VCN will
continue to provide networks, but not restrict underlying networks.
Similarly, this change allows for soft-starting of the VCN, where VCN
underlying networks can be selected without restricting them directly.

Additionally, this change ensures networks are torn down when
VcnGatewayConnections enter safe mode. This change is required due to
changes in the lifecycle of the VcnGatwayConnection, where in safe mode
they are NOT torn down, but allowed to continue retrying. During these
broken-connectivity windows, the VCN network should be torn down to
prevent blackholing traffic.

Bug: 183174340
Test: atest FrameworksVcnTests
Change-Id: I50f2c0e92552281731c843db89e9a9a1ccff5346