Fix WindowInsetsBehaviorTests failure

The system provides extra swipe start threshold when swiping
down from top in devices with a cutout at top.

We should include this extra area when reporting the system gesture
insets.

Bug: 196001869
Test: atest WindowInsetsBehaviorTests
Change-Id: I83a97be6cd7f2d13318d75aa26b9bf1ef3c1c428