GestureNav: Fix broken exclusion rect calculation for modal windows

Fixes an issue with non-fullscreen modal windows (such as dialogs), where the touch exclusion of
the window behind was still being applied, even though the window behind did not actually
receive the touches in the exclusion, because they would go to the modal window in front.

Bug: 135522625
Test: atest testCalculateSystemGestureExclusion_modal
Change-Id: Ia99f4f601e780715abaf966f6f297fd9d555fd0b