Initial implementation of AccountSettings. (#6275)

* Make the UserPreference macro also generate an accessor for the key name.

* Initial implementation of AccountSettings.

- Add an AccountUserDefaults storage with a userID prefix on the key.
- Migrate the 2 values reset by AppSettings.resetSessionSpecificSettings to AccountSettings.

* Rename UserSettings.volatile() to UserSettings.mock()

Given it now has a userID parameter with a default value..

UnitTests/Sources/LiveLocationManagerTests.swift

@@ -228,7 +228,7 @@ final class LiveLocationManagerTests {
     }
     
     private func setUp(accuracyAuthorization: CLAccuracyAuthorization = .fullAccuracy) {
-        userSettings = UserSettings.volatile()
+        userSettings = UserSettings.mock()
         clientProxy = ClientProxyMock(.init())
         beaconInfoSubject = PassthroughSubject<LiveLocationOwnInfoUpdate, Never>()
         clientProxy.liveLocationOwnInfoUpdatesPublisher = beaconInfoSubject.eraseToAnyPublisher()