Split apart "always show bookmarks toolbar" and the bookmarks toolbar display mode

[bsclifton]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Split apart "always show bookmarks toolbar" and the bookmarks toolbar display mode

Fixes #4936

Auditors: @bbondy

Test Plan:

1. Launch Brave and open Preferences
2. Verify you can toggle "always show bookmarks toolbar" and not have the drop down above be affected

[bbondy]

I think it might actually make the most sense to have 4 options:
Do not show bookmarks toolbar
Show bookmarks as text only
Show bookmarks as text and icons
Show bookmarks as icons only

/cc @bradleyrichter I think this is an improvement so I'll merge. Mainly it's really confusing seeing a disabled item with an option of "Text only" under bookmarks toolbar. And an option to control it underneath it.

But please advise if you'd like a followup to combine into one drop down which I think is the most clean.