Animate scrolling grid into place when there is a gap between last tasks

and clear all.

- Stagger animation starts
- Adds a grid translation property to TaskView for animating these
translations.

Also fixes an issue where the gap between clear all and focused task is
greater than spacing between grid tasks and clear all.

Test: manual
Fix: 188793333
Change-Id: Ib2ba8b1b84dc63c4ba186bd0b9b4962d3c66ce5a