Implement VCN error callback use.

This CL updates VcnGatewayConnection to notify VcnManagementService
(through Vcn) when errors occur with a gateway. VcnManagementService
then notifies registered, permissioned VcnStatusCallbacks via

Bug: 163433613
Test: atest FrameworksVcnTests
Change-Id: I3be3cac4b591b19a0b0075767fde0ba2eb6e12a2