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/IdentityConfirmationScreenViewModelTests.swift

@@ -109,7 +109,7 @@ struct IdentityConfirmationScreenViewModelTests {
         userSession.sessionSecurityStatePublisher = securityStateSubject.asCurrentValuePublisher()
         
         viewModel = IdentityConfirmationScreenViewModel(userSession: userSession,
-                                                        userSettings: .volatile(),
+                                                        userSettings: .mock(),
                                                         userIndicatorController: UserIndicatorControllerMock())
     }
 }