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

Simplify the logic of determining opaque and color of the navigation bar
by caching the window that is overlapping with the navigation bar.
Remove mForcingShowNavBar and mForcingShowNavBarLayer which are no
longer needed.

Bug: 179251684
Test: atest DisplayPolicyTests

Change-Id: Id5015014aff8db833f42e153a2f62fd999eca536