Skip writing item during shutdown process if the user is locked.

Otherwise the persist thread would keep trying to write that item,
may causing an infinite loop until timeout.

Flag: com.android.window.flags.record_task_snapshots_before_shutdown
Bug: 389571348
Test: run forrest with multi-user environment.
Change-Id: Ifeefbba2aeff3a13790e8366c5ac34cd62b7fea5