Update identification for onGatewayConnectionError().

This CL updates the identification method for
VcnStatusCallback#onGatewayConnectionErrror. Previously,
GatewayConnections were identified by an int[] specifying the
GatewayConnection's exposed NetworkCapabilities. Following API Council
feedback, this is updated to identify GatewayConnections by a
caller-provided String set in the VcnGatewayConnectionConfig.Builder.

Bug: 182345902
Bug: 180522464
Test: atest FrameworksVcnTests
Change-Id: I933c2330edb9bfc1b6bb62276debac02460e24f8