Improve UI for notification permission screen in onboarding

[jmartinesp]

Type of change

• Feature
• Bugfix
• Technical
• Other : improve UI.

Content

• Add OnboardingBackground component.
• Add background slot to HeaderFooterPage.
• Make the notification opt-in screen follow the latest Figma designs.

Motivation and context

Closes #2581.

Screenshots / GIFs

Before	After

Tests

On an Android 13+ device:

• Log into an account.
• Go through the verification step.
• Check the new notification screen.

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 includes a new file under ./changelog.d. See https://github.com/element-hq/element-x-android/blob/develop/CONTRIBUTING.md#changelog
• 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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[codecov]

Codecov Report

Attention: Patch coverage is 91.11111% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 73.50%. Comparing base (192a1d2) to head (751c481).
Report is 1 commits behind head on develop.

Files	Patch %	Lines
...c/main/kotlin/io/element/android/x/MainActivity.kt	0.00%	1 Missing ⚠️
.../ftue/impl/notifications/NotificationsOptInView.kt	75.00%	0 Missing and 1 partial ⚠️
...ries/designsystem/atomic/pages/HeaderFooterPage.kt	92.30%	0 Missing and 1 partial ⚠️
...es/designsystem/components/OnboardingBackground.kt	96.29%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2660      +/-   ##
===========================================
+ Coverage    73.48%   73.50%   +0.01%     
===========================================
  Files         1456     1457       +1     
  Lines        35113    35142      +29     
  Branches      6733     6735       +2     
===========================================
+ Hits         25804    25832      +28     
  Misses        5793     5793              
- Partials      3516     3517       +1     

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[ganfra]

I've not checked the figma, so I don't know if paddings are ok but otherwise LGTM

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarCloud