Feature/tab switcher improvements

[CDRussell]

Task/Issue URL:

1. https://app.asana.com/0/488551667048375/795610899417598
2. https://app.asana.com/0/488551667048375/796714078222032
3. https://app.asana.com/0/488551667048375/795210608999753

Description:
Fixes a bug with the tab switcher where it sometimes fails to close the last remaining tab.

Additionally, adds the option to close all tabs (without 🔥all the data).

Finally, animates in the toolbar when you select to open a new tab in background if the toolbar is not already visible. This ensures the new BG tab animation is always seen by the user.

Steps to test this PR:

Tab switcher bug

1. Add non-background tab from SERP
2. Click the tab switcher
3. Close the first tab (not currently selected)
4. Attempt to close the remaining one; it should now be removed

Close all tabs option

1. Add multiple tabs
2. Click the tab switcher
3. Use the overflow menu to close all tabs; verify they all disappear

Always show toolbar

1. Load the SERP (or any scrollable webpage)
2. Scroll down so that the toolbar disappears
3. Long press a link and choose to open tab in background
4. Verify the toolbar appears and the new tab animation is visible

---

Internal references:

Software Engineering Expectations
Technical Design Template

[CDRussell]

Fixes #334

[nilnilnil]

👍 pending product review.