Fix SnapshotTaskTests for cases where bars dont hide

- on Automotive, OEMs can choose to decline the immersive mode request
  and not hide the bars. Hence the screenshot should exclude the area
  overlapping with the system bars.

Fix: 344293643
Test: atest SnapshoTaskTests (with
      remote_insetsControllerControlsSystemBars = true) and bars not
      hiding
Change-Id: I9a101c5bd87607449a957ffc7a9477a19080a4fe