Add warning when adding a caption.

[bmarty]

Content

Add a warning picto (under an enabled FF) to let the user know that caption may not be visible for other users.
I prefer to keep the other FF (which enable or disable caption creation) but it is now enabled by default.

Motivation and context

Android friend of element-hq/element-x-ios#3574
Figma: https://www.figma.com/design/Ni6Ii8YKtmXCKYNE90cC67/Timeline-(new)?node-id=1990-23220

Screenshots / GIFs

Caption.warning.mp4

Tests

• Send an image and during the preview, see the new picto in the composer, which can be clicked

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 SonarQube Cloud

[codecov]

Codecov Report

Attention: Patch coverage is 95.87629% with 4 lines in your changes missing coverage. Please review.

Project coverage is 83.10%. Comparing base (b73884c) to head (c926697).
Report is 15 commits behind head on develop.

Files with missing lines	Patch %	Lines
...ent/android/libraries/textcomposer/TextComposer.kt	95.34%	1 Missing and 1 partial ⚠️
...ibraries/textcomposer/CaptionWarningBottomSheet.kt	96.29%	1 Missing ⚠️
...ibraries/textcomposer/model/MessageComposerMode.kt	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3977      +/-   ##
===========================================
+ Coverage    83.03%   83.10%   +0.07%     
===========================================
  Files         1796     1806      +10     
  Lines        45600    45703     +103     
  Branches      5375     5381       +6     
===========================================
+ Hits         37864    37983     +119     
+ Misses        5855     5832      -23     
- Partials      1881     1888       +7     

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