Make system bars appearence logic more generic (2/N)

Simplify the logic of determining color and opacity of navigation bar by
caching windows that are overlapping with the navigation bar.
Remove unused mForcingShowNavBar and mForcingShowNavBarLayer.

Bug: 179251684
Fix: 191383345
Test: atest DisplayPolicyTests
Test: atest NavigationBarColorTest
Test: verified with test actiivty that expanding IME panel and showing
dimming window at the same time.

Change-Id: I13d807d84ba85040058f7943e4652da8dc9fcca4