Ensure DisplayPolicyInsetsTests Check Correct Nav Bar Position

[1] introduced different navigation bar behavior
on displays with smallest width (sw) >= 600dp or with gesture nav
enabled, where the navigation bar would stay fixed to the bottom
when the display rotates to landscape/seascape. To ensure that
DisplayPolicyInsetsTests do not fail in these cases, modify the
tests to check the correct position navigation bar.

[1] I770cf207c08ba0d36de6e3b6ab378e94498a02bd

Bug: b/182499175
Test: atest DisplayPolicyInsetsTests

Change-Id: I9a84f25838166a343a8ed9e17a1e1b6584db574b