Fix a case that task jump when dismissing

- Fixed an edge case that RecentsView was not snapped to last tasks, but become snapped to last tasks after dismiss, as last pages' scroll is the position where ClearAllButton is barely invisible, instead of aligned to mLastComputedTaskSize.
- Fixed a case that misclassifed as split select

Fix: 202941373
Test: manual
Change-Id: Ib50dda471189da4325cd8ed57d600cc64907c978