Fix how task info is retrieved for split screen

Previously this was grabbing the top two tasks from the
task list, but this isn't guaranteed to be what's on the
screen. Instead, when already in split, use the taskInfo
from the split controller.

Test: manual - 1) have stuff in split
               2) keep dragging different apps into split
               => verify the app icon shown in the non
                  highlighted side matches the app on that
                  side
Bug: 209504662
Change-Id: Ib959f44bc9eaebfb5e3f640d69cb6f3ddccedfcf