fix(window magnification): fix changeMagnificationSize_expectedWindowSize presubmit error

The window magnifier size calculation In WindowMagnificationControllerTest#changeMagnificationSize_expectedWindowSize is not consistent with the logic in WindowMagnificationController. Therefore, in some cases the result size would not be the same and cause the failing. Accordingly, we fix the calculation in WindowMagnificationControllerTest to ensure the result is consistent.

Bug: 306201035
Bug: 307486608
Flag: NA
Test: atest WindowMagnificationControllerTest locally & test on abtd
Change-Id: I3f61eef32ac31bb55d678e02200102caf0c73b68