[flexiglass] Fix LOCK_ON_USER_SWITCH_CALLBACK not being called

KeyguardServiceShowLockscreenInteractor.onKeyguardServiceDoKeyguardTimeout() ensures the respective callback is added, but wasn't executed on flexiglass (SceneContainerFlag.isEnabled()).

Fixes: 412923724
Flag: com.android.systemui.scene_container
Test: manual test - switch user via login app with flexiglass enabled
Change-Id: Id324022337e8c26aa1714c977f3d47cb1fd41482