Use grid Rect when calculating clear all position

- This is a follow-up of http://ag/13827968 that uses separate grid Rect to calculate grid layout position
- This fixes the case when there are not enough tasks to fill the screen

Bug: 174464863
Test: Launch overview grid with only one task, clear all should not be shown until scrolled
Change-Id: I4c4f720bc83e6f1feca83285adbe567deea2d063