Cleanup and fixtures for SDK classes.

[bmarty]

Content

Replacement for #3482 : revert change about interface, but keep the cleanup and move fixtures in a dedicated packages.

Motivation and context

Clean code.

Screenshots / GIFs

Tests

• Step 1
• Step 2
• Step ...

Tested devices

• Physical
• Emulator
• OS version(s):

Checklist

• Changes have been tested on an Android device or Android emulator with API 23
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/element-hq/element-x-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request title will be used in the release note, it clearly define what will change for the user
• Pull request includes screenshots or videos if containing UI changes
• Pull request includes a sign off
• You've made a self review of your PR

[github-actions]

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.67%. Comparing base (df72d28) to head (352dbdb).
Report is 7 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3488   +/-   ##
========================================
  Coverage    82.67%   82.67%           
========================================
  Files         1731     1731           
  Lines        40834    40834           
  Branches      4967     4967           
========================================
  Hits         33760    33760           
  Misses        5312     5312           
  Partials      1762     1762           

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