Define VcnStatusCallback#onGatewayConnectionError callback.

This CL updates VcnStatusCallback to have a onGatewayConnctionError()
function. This function will be used to notify callers when Gateway
Connection errors occur, such as authentication failures or the session
unexpectedly dying.

Bug: 163433613
Test: atest FrameworksVcnTests
Change-Id: I9ea6e9e850eb9d8b0d374c5447895f1f04121696