For #4947 - Add Fenix Beta icons.

[liuche]

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This is also part of #109, to get our closed Firefox Preview Beta channel up and running. It looks like the releng bits have landed, so let's get these icons in too.

Did a quick test by building this with geckoBetaFenixBeta variant, saw the icons.

Pull Request checklist

• Quality: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
• Tests: This PR includes thorough tests or an explanation of why it does not
• Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features

After merge

• Milestone: Make sure issues finished by this pull request are added to the milestone of the version currently in development.

To download an APK when reviewing a PR:

1. click on Show All Checks,
2. click Details next to "Taskcluster (pull_request)" after it appears and then finishes with a green checkmark,
3. click on the "Fenix - assemble" task, then click "Run Artifacts".
4. the APK links should be on the left side of the screen, named for each CPU architecture

[boek]

This may need the changes that landed in master today to fix the build failures

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@40cda1d). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             master   #6173   +/-   ##
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  Coverage          ?   14.3%           
  Complexity        ?     326           
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  Files             ?     261           
  Lines             ?   10756           
  Branches          ?    1572           
========================================
  Hits              ?    1539           
  Misses            ?    9090           
  Partials          ?     127

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