Add a test for updateLightNavigationBarLw()

As a preparation to allow IME windows to use
SYSTEM_UI_FLAG_LIGHT_NAVIGATION_BAR, this CL makes
PhoneWindowManager#updateLightNavigationBarLw() testable and adds
several test cases to clarify how this method currently works.

This is a mechanical change and there should be no behavior change.

Bug: 69002467
Test: atest com.android.server.policy.PhoneWindowManagerTest
Change-Id: I2761cc037b013da52507592369a33e86a3138c14