Shields notification number behavior needs updated

[rossmoody]

The Shields icon number tally in the toolbar needs a behavior updates to be visible 100% of the time in both Off and On states. The current behavior is for the number to disappear entirely when shields is Off or not blocking anything. This behavior should be updated so when no ads are being blocked we will show a 0 indicating that no ads are currently being blocked. This goes for both off and on states.

[rossmoody]

Getting ducks in a row, reviewing as a larger team and will circle back with more concise direction on Brave Action item behavior/styling.