ThemeOverlayController: Don't enable neutral overlay when we use dark bg

Slightly hacky, but this allows the dark background to be applied to more 3rd party apps

Conflicting values in neutral overlays are:
color/system_neutral1_900
color/system_neutral2_900

Will consider only removing these from the overlay instead of disabling the whole thing

Change-Id: Icca896c15d169fc6e87f89cef4b5677c23daf5fb
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>