If the rootTaskIdsToRestore is empty...

on user switch, restore the home task to the top for the user. This
happens when the home task belongs to the system user and is visible to
a secondary user.

Test: atest RootWindowContainerTest & manual
Bug: 391501236
Flag: com.android.window.flags.enable_top_visible_root_task_per_user_tracking
Change-Id: I345dc1b965cc21e5cbb12a6ee4dade79927eb696