Fix wrong split percentage after split layout been pushed up

The task thumbnails of splitting task might not start with (0, 0), like
the case when IME panel pushing up split layout. Updated to consider
different start position while calculating split percentage.

Fix: 218704243
Test: enter split in portrait, toggle on IME panel, observed task
      thumbnails been placed properly after swiped up to overview.
Change-Id: I5b0d9710b56e9d77b86369c7fff4bc3ca75d9c7e