Make sure nav bar has height in test display

In TestDisplayContent, it set the hasNavigationBar flag to true when the
test needs one. However, this may not has the correct configuration when
the navigation bar height is criticle, e.g., when the screen is in
square.

The new flexible insets won't have any bar height if there's no bar,
even if the flag says there's one. To make the tests work as before,
make the bar height method always return a value.

Test: atest ActivityRecordTests#testCloseToSquareFixedOrientationLandscape
Test: atest DisplayConfigTests
Test: atest TaskLaunchParamsModifierTests
Bug: 194763108
Change-Id: Ife259c106f4ae7c7dbb6baebd26f8f98037c898a