For #17963 - Present Fenix as a browser app to the system

[Mugurell]

Fenix will now be able to be started from a call to
Intent.makeMainSelectorActivity(Intent.ACTION_MAIN, Intent.CATEGORY_APP_BROWSER)

For more info:
https://developer.android.com/reference/android/content/Intent#CATEGORY_APP_BROWSER

While Fenix will now appear in the list of browsers it still doesn't know open
links coming from ACTION_MAIN Intents. Needs some AC modifications first - mozilla-mobile/android-components#9805

FlipboardOpeningFenix.mp4

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.

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

[codrut-topliceanu]

LGTM! 🥇