Closes #2770: Allow receiving tabs from FxA devices

[grigoryk]

Now that we're on a-c 0.54, we can land this since it supports device
capability migration.

This patch adds a SEND_TAB device capability, making Fenix a valid target
in the Send Tab device list on Desktop Firefox.

Additionally, it adds a notification manager which manages notification
channels and knows how to display "received tabs" notifications".

Currently, device events are refreshed every 15 minutes in the background worker.
Additionally, they can be polled manually via "Sync Now" in account settings.

Once push integration lands as part of #246, tabs will be received nearly instantly.

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

The part that could benefit from unit tests here is the notification manager, but I'm not sure how to unit test it.

• Changelog: This PR includes a changelog entry or does not need one
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features

Notifications are plaintext, which I'm assuming will play nicely with the screen readers.

Screenshots

Receiving multiple tabs

App notification system settings (i.e. how strings added in this PR will appear):

[grigoryk]

In the screenshots you'll notice that the notification is missing the icon (to the left of the app name). I'm not sure why that is - I'm specifying ic_notification, which seems like a valid icon..?