Notify Vcn of VcnGatewayConnection quits

This change updates the Vcn when any GatewayConnection StateMachine
quits, removing it from the map of active GatewayConnections, and
triggering a re-evaluation of all existing NetworkRequests. This may (in
the event of a updated policy) result in an immediate restart.

Bug: 179944275
Test: atest FrameworksVcnTests
Change-Id: I72abf1e270a772760a17a03173d5a110e15bd01d