Fix NMS.enqueueTextToast() for user visibility.

When a visible background user toasts, the toast should be displayed
on the display associated to the user, but currently its shown in the
default display.

This change partially fixes that by blocking the toast - the full fix
(i.e., showing the toast in the right display) will come next...

Test: atest NotificationManagerServiceTest SnoozeHelperTest
Test: atest --user-type secondary_user_on_secondary_display android.widget.cts ToastTest#testShow_whenTextToast,testShow_appContext_whenTextToast
Test: atest android.widget.cts ToastTest#testShow_whenTextToast,testShow_appContext_whenTextToast # to make sure the "normal" case still works

Bug: 273314058

Change-Id: If7c789500460eff822946d24ba093e3b9c7a435c