Add permission for avoiding toast rate limiting.

I've decided to use a new permission since it needs to be added to
the settings app, which doesn't have INTERNAL_SYSTEM_WINDOW permission,
so I didn't want to add it as it may enable it to do some other things
it's not supposed to do.

Bug: 182448615
Test: atest android.widget.cts.ToastTest as regression test
Test: atest NotificationManagerServiceTest
Change-Id: I6adc84856f1fe9c33e0ad8470e7248a20587a1e9