* Add prefs for each local search shortcut
* Remove the `update2.localOneOffs` pref since it's not necessary with the three
new prefs
* Modify preferences UI by adding a new row in the engines tree for each local
shortcut
* Add `UrlbarUtils.LOCAL_SEARCH_MODES` so we have a single place where local
search modes and their properties are defined
* Add a new test file for the preferences UI
* Modify browser_oneOffs.js to test the three new preferences
Differential Revision: https://phabricator.services.mozilla.com/D97376