Use the crop function in the surfaceflinger screenshot api.

- By having surfaceflinger crop the screenshot instead of
  WindowManagerService, this greatly reduces the size of
  of temporary screenshot buffers that tablets have to
  allocate.
- App to home, App to recents times are improved noticeably.

Change-Id: Iff889cc8c57b157778fb16993a71546fd9abfc00
Bug 17006948
Bug 16987565