[Minor] Make Constants final

[kaijchen]

What changes were proposed in this pull request?

1. Make Constants class final.
2. Make public static fields in Constants final.
3. Make default constructor of Constants private.

Why are the changes needed?

Code quality.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Existing CI.

[zuston]

LGTM

[codecov-commenter]

Codecov Report

Merging #501 (c44df83) into master (9646a6f) will increase coverage by 0.75%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #501      +/-   ##
============================================
+ Coverage     59.34%   60.09%   +0.75%     
+ Complexity     1747     1615     -132     
============================================
  Files           206      192      -14     
  Lines         11508    10224    -1284     
  Branches       1030      908     -122     
============================================
- Hits           6829     6144     -685     
+ Misses         4274     3705     -569     
+ Partials        405      375      -30     

Impacted Files	Coverage Δ
...pache/hadoop/mapreduce/task/reduce/RssFetcher.java
...n/java/org/apache/hadoop/mapreduce/RssMRUtils.java
...pache/hadoop/mapreduce/task/reduce/RssShuffle.java
.../java/org/apache/hadoop/mapreduce/RssMRConfig.java
...java/org/apache/hadoop/mapred/SortWriteBuffer.java
...apache/hadoop/mapreduce/v2/app/RssMRAppMaster.java
...n/java/org/apache/hadoop/mapreduce/MRIdHelper.java
...mapreduce/task/reduce/RssInMemoryRemoteMerger.java
...rg/apache/hadoop/mapred/RssMapOutputCollector.java
.../hadoop/mapreduce/task/reduce/RssBypassWriter.java
... and 3 more

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[kaijchen]

Thanks @zuston for the review.