Limit the number of posted toasts by package to 5.

We consider posting a toast many seconds after it was triggered bad UX,
this change limits that behaviour.

Test: atest NotificationManagerServiceTest
Test: atest android.widget.cts.ToastTest
Bug: 171959286
Change-Id: I5d8cded37fbc9e5d4dca195436e1dff81c7f0e9e