Reducing the number of recent tasks we keep.

- This should decrease the time it takes to fetch the recent task list,
  and also reduce the number of tasks we store on-disk.
- Since a user can still open a fair number of tasks within the 
  time-window, and this list includes tasks that are excluded from the 
  UI, we are just reducing it to about half for the time being.

Bug: 28318359
Change-Id: Ie21b3d3d09a567007015fbbecaf914123c71cc06