For #12002: Show default tab background when thumbnail not available

[jonalmeida]

Adds a default thumbnail background when we do not have one created yet (specifically after a migration).

Pull Request checklist

• 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. In addition, it includes a screenshot of a successful accessibility scan to ensure no new defects are added to the product.

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

[liuche]

rerunning test-debug for timeout: https://firefoxci.taskcluster-artifacts.net/eh0VCU3UQLaXUooAI5QEuw/0/public/reports/test/testGeckoNightlyDebugUnitTest/classes/org.mozilla.fenix.components.searchengine.FenixSearchEngineProviderTest.html#GIVEN%20sharedprefs%20contains%20installed%20engines%20WHEN%20installedSearchEngineIdentifiers%20THEN%20defaultEngines%20+%20customEngines%20ids%20are%20returned

[jonalmeida]

Random test failure: GIVEN sharedprefs contains installed engines WHEN installedSearchEngineIdentifiers THEN defaultEngines + customEngines ids are returned

[jonalmeida]

Filed #12042 for the failing intermittent test.

[jonalmeida]

Thanks liuche for restarting it, forgot to do that.