test(fix): update the test proxy's import paths to work with the shaded jar (#1561)

Author: liujiongxin

test-proxy/src/main/java/com/google/cloud/bigtable/testproxy/CbtTestProxy.java

@@ -49,10 +49,10 @@
 import io.grpc.ManagedChannelBuilder;
 import io.grpc.Status;
 import io.grpc.StatusException;
-import io.grpc.netty.GrpcSslContexts;
-import io.grpc.netty.NettyChannelBuilder;
+import io.grpc.netty.shaded.io.grpc.netty.GrpcSslContexts;
+import io.grpc.netty.shaded.io.grpc.netty.NettyChannelBuilder;
+import io.grpc.netty.shaded.io.netty.handler.ssl.SslContext;
 import io.grpc.stub.StreamObserver;
-import io.netty.handler.ssl.SslContext;
 import java.io.ByteArrayInputStream;
 import java.io.Closeable;
 import java.io.IOException;