Remove focus task

- Focus task is only removed under ENABLE_GRID_ONLY_OVERVIEW, but many fixes/refactoring are generic and apply with the flag off
- Keep moving running task to front if go to overvied after quick switch, so live tile is always on top right

Grid calculation:
- Fixed some top/bottom calculation assumption that assume focus task is always present

Size calculation:
- With ENABLE_GRID_ONLY_OVERVIEW, taskSize == gridTaskSize, and task is aligned to top-right of gridRect (instead of focusTaskRect)

Bug: 257952455
Test: Enter overview from home
Test: Enter overview from app, with variations that quick switch and enter
Test: Dismiss task from different position
Test: Split select task from different position
Test: Repeat with/without GRID_ONLY_OVERVIEW flag
Test: Repeat with handheld
Change-Id: I6580a8ac6c2c2059c33c4daca05b7d0354513e74