Avoid seeing ClearAllButton for overview grid's snap position

- This is a follow-up of http://ag/15220572
- When determining if task's snap position is too close to ClearAllButton, avoid positions that will see part of ClearAllButton

Bug: 192254835
Test: manual
Change-Id: I09425075ff0c24835ac3c4e79856aef7985476b2
1 file changed