Exit Safe Mode when a VCN gets new configs.

This CL updates VCN instances to exit Safe Mode when they receive a
new VcnConfig. When a VCN exits Safe Mode, they will re-register a
request listener to receive all cached requests and start the
needed VcnGatewayConnections.

Bug: 179429339
Test: atest FrameworksVcnTests
Change-Id: Id1c2dc287e1ec56f37821cf0ae77f066f22d22bb