Refactor the getSettings code to allow for dynamic default values

[bsclifton]

For reference, see

browser-laptop/js/settings.js

Line 40 in af4e7df

module.exports.getSetting = (settingKey, settingsCollection) => {

The code currently has a special hard-coded case for settingKey === settings.ACTIVE_PASSWORD_MANAGER. Instead of having an if statement (which executes for every call) this could be reworked to have a default value determined in resolveValue(). The code is already lookup up static values using the format config.settings[key] but currently doesn't have a mechanism for handling more dynamic values, like the logic found in passwordManagerDefault().

cc: @BrendanEich

[bsclifton]

Re-opening; issue was auto-closed when potential fix was submitted with #4640 (which never got reviewer feedback)