netty: Replace missed jsr305's GuardedBy with Error Prone's

This was missed in 70825ad

Author: ejona86

netty/src/test/java/io/grpc/netty/NettyClientTransportTest.java

@@ -46,6 +46,7 @@
 import com.google.common.base.Ticker;
 import com.google.common.io.ByteStreams;
 import com.google.common.util.concurrent.SettableFuture;
+import com.google.errorprone.annotations.concurrent.GuardedBy;
 import io.grpc.Attributes;
 import io.grpc.CallOptions;
 import io.grpc.ChannelLogger;
@@ -122,7 +123,6 @@
 import java.util.concurrent.TimeoutException;
 import java.util.concurrent.atomic.AtomicBoolean;
 import javax.annotation.Nullable;
-import javax.annotation.concurrent.GuardedBy;
 import javax.net.ssl.SSLException;
 import javax.net.ssl.SSLHandshakeException;
 import javax.net.ssl.TrustManager;