Save and pass last snapshot data with recents task info

- Adds task & snapshot sizing info to RecentTaskInfo so that SysUI can
  stably layout overview when there are tasks of different sizes
- Removing some unused code

Bug: 179466077
Test: atest WmTests:RecentTasksTest
Change-Id: Idf4c27e2d04380328413459fc537ff8bcc111a14