Fixes for touch exclusion region

- Evaluate touch exclusion region on per-task basis, instead of per
  window. Smallest unit for exclusion is Task, if task has more than
  one app window, we still need to use the visible area of the task
  to do focus transfer properly.

- Only add back the touch region for focused task after all tasks
  are processed for eclusion, otherwise the focused area could be
  removed from exclusion incorrectly.

- Skip app windows that's exiting or hidden.

bug: 25494928

Change-Id: I972ba2149431557e7420b1095d9496a6cd821bdb