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

[rxin]

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

[SparkQA]

Test build #54832 has finished for PR 12148 at commit 3e3d5ea.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #2738 has finished for PR 12148 at commit 3e3d5ea.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #2740 has finished for PR 12148 at commit 3e3d5ea.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[andrewor14]

nice

[SparkQA]

Test build #2741 has finished for PR 12148 at commit 3e3d5ea.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[andrewor14]

Merged into master

[andrewor14]

FYI @zsxwing