Fix DisplayPolicy#isOverlappingWithNavBar

The function name didn't match its logic. It only worked when
win.mActivityRecord is not null.

This CL fixes the logic and make the dimming window and the window with
TYPE_VOICE_INTERACTION be able to be mNavBarColorWindowCandidate.

Fix: 270298341
Test: atest DisplayPolicyTests
Change-Id: If216cf2f73615d82129e51fbacc326fd02cbb8f8