Render caller avatar on Incoming call screen

[bmarty]

Content

Ensure that the image loader is set, else the IncomingCallActivity will not be able to render the avatar.

Note that this is a temporary fix, we may want to revisit how we handle the Coil image loader singleton.

This PR also ensure that a background color is set else we can end up with light text on light background.

Motivation and context

Fixes #4633
Fixes #4629

Screenshots / GIFs

Before	After

Tests

• Set the system color to light
• Set the app theme to dark
• Kill the app
• Lock the device (screen is off)
• Receive a call from a DM where other user has an avatar
• Observe that the screen now renders the caller avatar and is rendering light text on dark background. Previously it was only rendering initials avatar, with light text on light background.

Tested devices

• Physical
• Emulator
• OS version(s):

Checklist

• 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 define 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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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.99%. Comparing base (76e1612) to head (3e0cf90).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4635   +/-   ##
========================================
  Coverage    79.99%   79.99%           
========================================
  Files         2107     2107           
  Lines        55811    55818    +7     
  Branches      6954     6955    +1     
========================================
+ Hits         44644    44651    +7     
  Misses        8772     8772           
  Partials      2395     2395           

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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