Prevent search-bar from being auto-hidden when not used for awhile

Lock the browser.search.widget.lastUsed pref to prevent disuse from hiding the search bar.

In Bugzilla 1866616 Mozilla now hides the search bar after 120 days of disuse. Since this bar is hidden by default, a user would have had to manually it the toolbar. We should disable this feature so that a user's UI isn't changed just because they haven't used the browser in 4 months.