Add trace logs for UserController#dispatchUserSwitchComplete.

To track how long it takes for each mUserSwitchObservers to process
IUserSwitchObserver#onUserSwitchComplete call and when that happens.

Bug: 239497256
Test: atest MultiUserPerfTests:android.multiuser.UserLifecycleTests#switchUser_stopped --no-bazel-mode
Change-Id: I9c618b6bb2247d93f624a8f9b9351608d4579000