alts: fix lazychannel close

Author: jiangtaoli2016

alts/src/main/java/io/grpc/alts/internal/AltsProtocolNegotiator.java

@@ -297,7 +297,7 @@ synchronized Channel get() {
     /** Returns the cached channel to the channel pool. */
     synchronized void close() {
       if (channel != null) {
-        channelPool.returnObject(channel);
+        channel = channelPool.returnObject(channel);
       }
     }
   }