Change password manager prefs UI to simple slider for Brave passwords

[bradleyrichter]

Now that we have an extensions manager, we no longer need to pre-list password managers in the password section of the privacy prefs panel.

This will allow users to combine multiple managers if desired, however complex that may be. It's up to them. (rather than limiting it to one manager at a time)

cc @cezaraugusto

[bsclifton]

I think this is too big for 0.14.2- moving to 0.14.3

[rebron]

Closing wontfix. Tracking overal pref changes in brave-core brave/brave-browser#955