Fix proguard config for nightly and release builds

[jmartinesp]

Content

Add a proguard rules file for the pushprovider firebase module.

Motivation and context

For some reason it seems like this module needs extra proguard rules.

Tests

Nightlies are working again.

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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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.56%. Comparing base (20d0278) to head (f9e3a67).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3294   +/-   ##
========================================
  Coverage    82.56%   82.56%           
========================================
  Files         1662     1662           
  Lines        38992    38992           
  Branches      4724     4724           
========================================
  Hits         32192    32192           
  Misses        5133     5133           
  Partials      1667     1667           

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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]

OK, thanks.