[STREAMING] [MINOR] Keep streaming.UIUtils private

zsxwing

Author: Andrew Or <andrew@databricks.com>

Closes #6134 from andrewor14/private-streaming-uiutils and squashes the following commits:

225df94 [Andrew Or] Privatize class

streaming/src/main/scala/org/apache/spark/streaming/ui/UIUtils.scala

@@ -19,7 +19,7 @@ package org.apache.spark.streaming.ui
 
 import java.util.concurrent.TimeUnit
 
-object UIUtils {
+private[streaming] object UIUtils {
 
   /**
    * Return the short string for a `TimeUnit`.