Add getAdjacentTaskFragment check for the split screen case

Previously, the root task created by kids mode would be treated the same
as split screen to return SCREEN_ORIENTATION_UNSPECIFIED.

Now, also check getAdjacentTaskFragment != null for the split
screen case, so that apps in the kids mode root task will work as
expect.

Bug: 231575466
Test: - Enter kids mode -> rotate device
        - apps should follow their request orientations
      - Enter split screen(not in kids mode) -> rotate device
        - tasks should be rotated
Change-Id: I930e162ef74d114ba2375eae03e13f7ccdd34baa