Force app to draw navigation bar background

For gestural navigation, having a small bar drawn by SystemUI
looks really really weird. Since we force the status bar
background to be drawn in the client, we can also force the
navigation bar background without expecting any compat issues.

Bug: 129751541
Test: Open legacy app, ensure black is drawn in the client
Change-Id: I9cd4c5dd799324ff10bf9d28750e8f238ab1e459
(cherry picked from commit 461bb11371e956ab1ab2e19940d393d9d729be54)