Add config.xml fallback in DisplayDeviceConfig

This change adds the default brightness to the ddc, and a fallback to
config.xml when the ddc doesn't exist.

It adds the minimum and maximum brightness constrtaints to the display
device config - which are currently sourced from config.xml.

Bug: 147415200
Test: manual

Change-Id: Ibbfbbbd495048114befb3f867bd5f4e26916ca9e