Don't try to show the current toast again while it's showing.

By doing this we avoid a few bad things:
 - mechanism that hides the current toast by trying to show it again
 - delaying the call to hide and remove the current toast from the queue
   when it's duration expires (which in the case of repeated calls can
   delay this indefinitely)

Test: atest NotificationManagerServiceTest
Test: atest android.widget.cts.ToastTest
Bug: 167672740
Change-Id: Ie4953109314113efae49fa0c5e0c236e6e0dbb23