Add tests for `A5Cipher.java`, improve class & function documentation

[Hardvan]

trafficstars

• [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 56.85%. Comparing base (93cfa86) to head (94c7648).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5594      +/-   ##
============================================
+ Coverage     56.80%   56.85%   +0.05%     
- Complexity     3600     3601       +1     
============================================
  Files           545      545              
  Lines         15723    15723              
  Branches       2995     2995              
============================================
+ Hits           8931     8939       +8     
+ Misses         6415     6406       -9     
- Partials        377      378       +1     

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[Hardvan]

Thanks!