Fix rotation test faiulre on foldable devices

In our test, we assume the device is under portrait mode, which is not
true for foldables.

To fix it, we caclue the rotation degree based on the current rotation.

Bug: 197623044
Test: run WindowMagnificationControllerTest with the device under the
landscape mode.

Change-Id: I4c27a193af05f402246aa8b3b490e8bde9d93fc7