Improve pip and add feature flag.

[bmarty]

Content

Add feature flag for PiP, disabled in production for now.
Change Call screen top left icon, when pip is supported, use a Back arrow instead of a Close. Closes #3197
Hang up when the Pip is closed.

Motivation and context

Improve Pip feature, and disable the feature until it is correctly tested and production ready.

Screenshots / GIFs

Tests

• Play with the call with and without activating the feature flag.
• When in PiP, close the window and observe that on the timeline the call is correctly hanged up.
• If Pip is supported and the FF is enabled, observe that the Call screen is displaying a Back button.

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

[bmarty]

I think this is OK to use a runBlocking here, since at some point the FF will be removed.

[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 SonarCloud

[codecov]

Codecov Report

Attention: Patch coverage is 47.61905% with 11 lines in your changes missing coverage. Please review.

Please upload report for BASE (develop@7ae5e99). Learn more about missing BASE report.
Report is 3 commits behind head on develop.

Files	Patch %	Lines
...droid/features/call/impl/pip/PipSupportProvider.kt	0.00%	6 Missing ⚠️
...droid/features/call/impl/ui/ElementCallActivity.kt	0.00%	4 Missing ⚠️
.../android/libraries/featureflag/api/FeatureFlags.kt	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #3199   +/-   ##
==========================================
  Coverage           ?   76.11%           
==========================================
  Files              ?     1643           
  Lines              ?    38686           
  Branches           ?     7480           
==========================================
  Hits               ?    29446           
  Misses             ?     5349           
  Partials           ?     3891           

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[jmartinesp]

LGTM, thanks!