[a11y] Improve accessibility of screen headers.

[bmarty]

Content

Improve accessibility of TopBar titles: Composable must have the heading() semantic property (as per the documentation).
Also group title and subtitle in some screens so they are read together.

Motivation and context

Closes #6387

Screenshots / GIFs

Tests

• Enable TalkBack
• Navigate to modified screens (space, media viewer, thread, thread list, room timeline) and observe that the top bars are declared as "header"

Tested devices

• Physical
• Emulator
• OS version(s):

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.27%. Comparing base (64d9336) to head (df9a3fe).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #6827   +/-   ##
========================================
  Coverage    81.26%   81.27%           
========================================
  Files         2648     2648           
  Lines        74171    74189   +18     
  Branches      9622     9622           
========================================
+ Hits         60278    60296   +18     
  Misses       10341    10341           
  Partials      3552     3552           

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[ganfra]

LGTM