Fix TaskView.getThumbnailBounds not combining bounds correctly if relativeToDragLayer is true

- This was a coding error introduced in ag/27382735
- getThumbnailBounds is supposed to union all individual thumbnailBounds per TaskContainer, rather than overriding bounds over and over per TaskContainer

Bug: 339787250
Test: Swipe up to home from split apps
Flag: EXEMPT bugfix
Change-Id: I7a697620f4b4507df8f1860b4c2f9889e4be006b