[MINOR] chore: update import order rule to make scala a separated group

[advancedxy]

What changes were proposed in this pull request?

1. update checkstyle.xml to make scala a separated import group
2. modify java files accordingly

Why are the changes needed?

Better code style

Does this PR introduce any user-facing change?

No.

How was this patch tested?

No need. There's no semantics change.

[codecov-commenter]

Codecov Report

Merging #818 (afc1f71) into master (cae7cd9) will increase coverage by 1.33%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #818      +/-   ##
============================================
+ Coverage     57.63%   58.96%   +1.33%     
+ Complexity     2058     2057       -1     
============================================
  Files           306      292      -14     
  Lines         14871    12899    -1972     
  Branches       1221     1221              
============================================
- Hits           8571     7606     -965     
+ Misses         5808     4861     -947     
+ Partials        492      432      -60     

Impacted Files	Coverage Δ
.../java/org/apache/spark/shuffle/RssSparkConfig.java	98.07% <ø> (ø)
...org/apache/spark/shuffle/RssSparkShuffleUtils.java	55.46% <ø> (ø)
...e/spark/shuffle/reader/RssFetchFailedIterator.java	0.00% <ø> (ø)
...e/spark/shuffle/reader/RssShuffleDataIterator.java	94.80% <ø> (ø)
...pache/spark/shuffle/writer/WriteBufferManager.java	85.78% <ø> (ø)

... and 15 files with indirect coverage changes

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