Define colorBackground for Theme.DeviceDefault.Settings.

Use device_default_light as colorBackground for Settings theme. The
color is a pass through to background_material_light. Having the
passthrough color make it easier for OEMs to define runtime theme
overlay for background color.

Bug: 69314327
Test: visual
Change-Id: I71cabbf43c20b88fc856cd5bba5a521011d56990