Revert app icons background update

[renanferrari]

This PR reverts the changes introduced by #14829.

Since this targets the frozen branch, we should wait for confirmation before merging it. This can be merged as soon as it is approved.

To test:

Build the vanilla variant of the app and verify that the icon has its original background back. Try it out with different screen resolutions and Android versions.

Regression Notes

1. Potential unintended areas of impact
   N/A

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   N/A

3. What automated tests I added (or what prevented me from doing so)
   N/A

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APK here.

[AliSoftware]

😢 so sad to see this icon go 😭