Make thresholds tuneable in the ddc

Add screen and ambient thresholds to the ddc.
Use these values as a minimum required change, so that brightness can be
more stable at lower light levels.

Bug: 205683328
Test: adb shell dumpsys display
Test: atest AutomaticBrightnessControllerTest

Change-Id: I3af6d754899d75ed51964ed7ad140f1572ec41b0