Let notification shade draw navigation bar background

If there is no app window showing underneath nav bar (e.g., the screen
is locked), let system windows (ex: notification shade) draw nav bar
background.

This fixes a regression since f6d661df

Fix: 284922450
Bug: 274666717
Bug: 284041258
Test: 1. Go to lockscreen in gesture nav mode and see if nav bar is not
         opaque.
      2. Go to the "Sign In" page of PressReader, show IME, and see if
         nav bar obscures the dismiss button of IME.
Change-Id: I9fea64f9bdd9b0142aca61ce6e68985c3557dab6