Place black background only behind nav bar when letterboxed

Placing black background behind the entire app window causes
alpha animation issues, that can be visible as black flash to
user.
This CL sizes and positions the background only behind the
navigation bar area.

Bug: 64598822
Test: Open letterboxed app, go to/from Recents, observe animations.
Change-Id: I16a2243117f87ca513d5ef291b7c1d3779467f3b