Restore all visible root tasks on user switch.

Instead of storing just the top focused task, store the visible tasks
for the user. When the user switch happens, restore these tasks from
bottom to top.

Test: atest RootWindowContainerTest
Flag: com.android.window.flags.enable_top_visible_root_task_per_user_tracking
Bug: 381038076

Change-Id: Ic1795d1492499484534edf43d2d07ec3eda8a8c8