Reevaluate VcnGatewayConnections on receiving new configs.

This CL updates Vcn.java to reevaluate its VcnGatewayConnections when it
receives a new VcnConfig. This may result in VcnGatewayConnections being
torn down (if their VcnGatewayConnectionConfig is not in the updated
VcnConfig) or established (if a NetworkRequest matches a new
VcnGatewayConnectionConfig).

Bug: 181815405
Test: atest FrameworksVcnTests CtsVcnTestCases
Change-Id: I7da60f60e972bd968e1ff4fe416fb9a1d3309a18