Ensure that the manual dark theme is rendering correctly regarding -night resource and keyboard

[bmarty]

Content

Ensure that the system is aware about "forced" dark theme.
Using AppCompatDelegate.setDefaultNightMode is actually working like a charm, I have not found any regression.

Motivation and context

Full dark theme when system setting is set to light and application setting is set to dark.

Closes #3168

Screenshots / GIFs

Before	After

Tests

• Set system theme to light
• Set application theme to dark
• Before: keyboard was light and some screens were broken (see Assets in -nightly folder are not used when theme is forced to "Dark" #3168)
• Clear cache from the application: observe that before the screen was quite broken (see below) and is OK now.

Try to change system theme and application theme and observe that the application is always rendering the expected theme.

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

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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

[bmarty]

Note that it could be done elsewhere, for instance in RootPresenter, but when the value is the same, setDefaultNightMode has no effect, so I guess it's fine to do it here.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 76.13%. Comparing base (0d41788) to head (5ca0f64).
Report is 3 commits behind head on develop.

Files	Patch %	Lines
...id/libraries/designsystem/theme/ElementThemeApp.kt	0.00%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3216      +/-   ##
===========================================
- Coverage    76.14%   76.13%   -0.01%     
===========================================
  Files         1645     1645              
  Lines        38809    38815       +6     
  Branches      7516     7517       +1     
===========================================
+ Hits         29552    29553       +1     
- Misses        5356     5362       +6     
+ Partials      3901     3900       -1     

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[ganfra]

LGTM!