Expose API for identifying GatewayConnections.

This CL exposes the updated APIs for identifying GatewayConnections via
a user-configured String set in VcnGatewayConnectionConfig.Builder, as
requested by the API Council.

Bug: 182345902
Bug: 180522464
Test: atest FrameworksVcnTests
Change-Id: I10afd074906bc0f3831157dcee1da813b4cfce78