Fix pinned events banner reappearing when loading

[jmartinesp]

Content

Change the RustTimeline.timelineItems property from a MutableStateFlow with an empty list to a MutableSharedFlow which will be populated with the contents of the Rust SDK timeline as soon as they're available.

Motivation and context

Fix an issue where the display animation happened twice when loading items because the first items loaded into the pinned events timeline were an incorrect empty list.

Tests

• Open a room with pinned events.
• The animation should run only once.

Tested devices

• Physical
• Emulator
• OS version(s): 14

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 (5c7ac76) to head (28a5105).
Report is 9 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3519   +/-   ##
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  Coverage    82.67%   82.67%           
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  Files         1731     1731           
  Lines        40863    40863           
  Branches      4971     4971           
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  Hits         33785    33785           
  Misses        5315     5315           
  Partials      1763     1763           

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[frebib]

Does this also solve the weirdness I see sometimes that causes the banner to slide in diagonally from the top-left instead of straight down?

[ganfra]

Does this also solve the weirdness I see sometimes that causes the banner to slide in diagonally from the top-left instead of straight down?

Yes it does!