style: make `SubsetCount` a proper utility

[vil02]

This change will allow to include MS_SHOULD_BE_FINAL. Continuation of #5122.

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
• [x] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [x] All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 38.25%. Comparing base (cb401fe) to head (5603bf7).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5153   +/-   ##
=========================================
  Coverage     38.24%   38.25%           
  Complexity     2361     2361           
=========================================
  Files           516      516           
  Lines         15310    15309    -1     
  Branches       2958     2958           
=========================================
+ Hits           5856     5857    +1     
+ Misses         9166     9165    -1     
+ Partials        288      287    -1     

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