Add admin policy for Web Discovery (#30023)
* Fix Web Discovery API to account for policy, add Web Discovery test
* Move IsWebDiscoveryEnabled/pref_names to components/web_discovery/common
* Check Web Discovery Native feature before loading extension background page
* Use PrefService ref in IsWebDiscoveryEnabled
* Mark WDP policy as pending development for Android
* Move Web Discovery enabled/disabled via policy prefs to //brave/components/constants