[SPARK-14364][SPARK] HeartbeatReceiver object should be private

## What changes were proposed in this pull request?
It's a mistake that HeartbeatReceiver object was made public in Spark 1.x.

## How was this patch tested?
N/A

Author: Reynold Xin <rxin@databricks.com>

Closes #12148 from rxin/SPARK-14364.

core/src/main/scala/org/apache/spark/HeartbeatReceiver.scala

@@ -220,6 +220,7 @@ private[spark] class HeartbeatReceiver(sc: SparkContext, clock: Clock)
   }
 }
 
-object HeartbeatReceiver {
+
+private[spark] object HeartbeatReceiver {
   val ENDPOINT_NAME = "HeartbeatReceiver"
 }