SystemUI: Fix nav bar transparency when enabling software keys

Repro:
 - Boot device with hardware keys
 - Go to Settings -> Buttons and enable on-screen nav bar
 - Press home
 - Observe launcher now does not have transparent nav bar

Ticket: CYNGNOS-2553
Change-Id: I49d04384b72b02ecb432be650d46e136c982a3bd
(cherry picked from commit 0053be36)