Only load full media on media viewer when it's the visible item

[jmartinesp]

Content

In the media viewer screen:

• Only load the full media for the currently displayed item.
• Cancel loading the full media for items if you scroll to another item.

Motivation and context

Fixes #6792.

What's not addressed but could also be useful:

• Loading the full media only when using Wi-Fi.
• Delay loading the full media for videos until the play button is clicked on.

Screenshots / GIFs

No UI changes, I hope.

Tests

Open the media viewer in a room where you have a mix of images and videos. Check the logs only contain /download log lines for the current item, and if you scroll fast enough you see logs for cancelled downloads.

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

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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

[codecov]

Codecov Report

❌ Patch coverage is 55.55556% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.28%. Comparing base (cbc677b) to head (59b8a81).

Files with missing lines	Patch %	Lines
...s/mediaviewer/impl/viewer/MediaViewerDataSource.kt	33.33%	4 Missing and 2 partials ⚠️
...es/mediaviewer/impl/viewer/MediaViewerPresenter.kt	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #6794      +/-   ##
===========================================
- Coverage    81.29%   81.28%   -0.01%     
===========================================
  Files         2648     2648              
  Lines        74122    74134      +12     
  Branches      9606     9614       +8     
===========================================
+ Hits         60255    60259       +4     
- Misses       10323    10328       +5     
- Partials      3544     3547       +3     

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