Fix the initialization of the system gesture top location

1. Initialize the system gesture top location in the constructor
2. Fix the calculation of system gesture top location when there is no
   botton inset

Bug: 380320995
Test: atest WindowMagnificationControllerTest
Test: atest WindowMagnificationAnimationControllerTest
Flag: com.android.systemui.update_window_magnifier_bottom_boundary
Change-Id: Ie6d97a6fe2be6b07ea9e1487f71d572f1d2fd397