Add fallback for sensor identification in ddc

If a display device config was specified but without a valid ambient light sensor, it would fall back to Sensor.TYPE_LIGHT rather than the specified one in config.xml. This change fixes that.

Bug: 187130675
Test: adb shell dumpsys display | grep DisplayDeviceConfig
Test: com.android.server.display
Change-Id: I5b3f71a7c32055dbbd43d8fd139cfa7d09b9b3c4