GestureNav: Fix broken split screen navbar

With gesture navigation, it no longer makes sense to force the navigation bar
to black in split screen, instead we can show the respective navbar backgrounds of
the applications.

Fixes: 130913061
Test: enable gesture navigation, enter splitscreen, observe that navigation bar backgrounds are properly drawn, also verify the same if IME is open.
Change-Id: If1464dd9cf0c5499c7c33caab06eb430fb59cb30