SystemUI: Make recents grid view accept more than 8 tasks

* SystemUI itself does not limit number of tasks to be shown in grid
  after fb44d212, and we have to handle
  more than 8 tasks in the task stack.

* Since the grid layout algorithm is prepared for only 8 tasks, we just
  drop any task out of this limit, i.e. only tasks with taskIndex =
  [max(0, taskCount-8), taskCount) will be shown.

Change-Id: I8715666397bfe2067735daaaabf2cae1f2600f43