Letterboxing: ensure bar contrast

Ensures that any bars that draw over a letterbox draw their own
background such that they always have contrast.

This fixes an issue, where if a light status / navigation bar app was
letterboxed, the bar would still draw dark icons over the now black
letterbox, making the icons unreadable.

To do so, splits the letterbox into a layout and an apply surface
changes phase.

Change-Id: Ia8afa3386d75d9a72434d701b867c3ebc35cc36f
Fixes: 72696928
Test: Open a letterboxed app with a white navigation bar, verify it is visible
Test: ApiDemos > App > Activity > Max Aspect Ratio > 1:1, verify navbar is visible