Adding minimal customizable background colour support.

This adds back minimal support from
https://googleplex-android-review.googlesource.com/#/c/1442940/1
because the bigger thing had to be reverted.

Bug: 31459804
Change-Id: I545d1f7639c9dada417e95b5da6290414d3de05d
diff --git a/core/res/res/values/colors_device_defaults.xml b/core/res/res/values/colors_device_defaults.xml
index 27ee27b..89691e9 100644
--- a/core/res/res/values/colors_device_defaults.xml
+++ b/core/res/res/values/colors_device_defaults.xml
@@ -32,4 +32,9 @@
     <color name="accent_device_default_light">@color/accent_material_light</color>
     <color name="accent_device_default_dark">@color/accent_material_dark</color>
     <color name="accent_device_default_50">@color/material_deep_teal_50</color>
+
+    <color name="background_device_default_dark">@color/background_material_dark</color>
+    <color name="background_device_default_light">@color/background_material_light</color>
+    <color name="background_floating_device_default_dark">@color/background_floating_material_dark</color>
+    <color name="background_floating_device_default_light">@color/background_floating_material_light</color>
 </resources>