alts: Use grpc-netty-shaded instead of grpc-netty

There's no good way to provide users of ALTS a choice between grpc-netty
and grpc-netty-shaded. Since Netty is not exposed through the ALTS API
surface, we opt for the shaded version as it has fewer deployment
issues. However, this also means that we _can't_ expose any Netty API,
like EventLoopGroup.