fix(magnification): draggable bounds of the settings panel is not correctly computed

The original computation of the settings panel dragging area bounds is not correct, so the settings panel would cover the gestural navigation bar area. It causes tapping on the bottom of the panel Done button would be also detected as tapping on the navigation bar, so it would trigger the window transition then make the fullscreen magnification zoom out. Therefore, we fix the computation to prevent the panel from covering the system bar area.

We also add test case in WindowMagnificationSettingsTest to verify the dragging bounds computation is expected.

Bug: 311319434
Flag: none
Test: manually
      atest WindowMagnificationSettingsTest
Change-Id: Iec81c544d3498ff02451b8ca1461c579846e52ac