Move empty day separator filtering to a timeline post-processor

[jmartinesp]

Content

A refactor of the code previously added in #6360.

This follows our current model better than applying the filtering in the RoomBeginningPostProcessor and then on the mapped timeline items.

Motivation and context

Better consistency with the existing code.

Tests

Everything should work as before, some unit tests were added, others simply moved.

Tested devices

• Physical
• Emulator
• OS version(s): 15

Checklist

• This PR was made with the help of AI:
  • Yes. In this case, please request a review by Copilot.
  • No.
• Changes have been tested on an Android device or Android emulator with API 24
• 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 defines what will change for the user
• Pull request includes screenshots or videos if containing UI changes
• You've made a self review of your PR

[github-actions]

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[sonarqubecloud]

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 SonarQube Cloud

[ganfra]

LGTM thanks!

[codecov]

Codecov Report

❌ Patch coverage is 90.32258% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.26%. Comparing base (f447560) to head (04dda8a).
⚠️ Report is 17 commits behind head on develop.

Files with missing lines	Patch %	Lines
...ssor/FilterPublicMembershipChangesPostProcessor.kt	80.00%	0 Missing and 2 partials ⚠️
...eline/postprocessor/FilterEmptyDayPostProcessor.kt	91.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #6866      +/-   ##
===========================================
- Coverage    81.26%   81.26%   -0.01%     
===========================================
  Files         2652     2654       +2     
  Lines        74404    74411       +7     
  Branches      9650     9650              
===========================================
+ Hits         60463    60467       +4     
- Misses       10378    10381       +3     
  Partials      3563     3563              

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