Add config flag to enable/disable nav bar auto dim

Added a configuration flag to enable whether to auto dim the navigation
bar when wallpaper changes to be invisible such as launching an opaque
app.

OLED devices should have this flag on to prevent/reduce burn and off for
everything else to avoid extra cpu cycles rendering an alpha animation.
By default this is on.

Test: manual
Change-Id: I57ab2a6414b5f6c34e11911346ad4c4d3aac33d0
Fixes: 68812918
(cherry picked from commit 4096e8d028da778f0afbda7df92d6de741650629)