Set top and bottom of lock icon to be global rect.

We no longer set the center when we migrate the lock icon. We can
achieve the same thing by getting the top and bottom of the global rect.

Test: Show ambient indication with flag on and off.
Test: Add a ton of notifications.
Fixes: 294871771
Change-Id: I113f5a3b14b2c48a1b415a03efac47a789a43a79