Ensure invalid brightness values aren't used

Previously, we were adding invalid brightness values to the short term
model; specifically, -1.0 was being added, causing the short term model
to be very dark. This change ensures that we validate the values before
adding them to the curve.

Bug: 283055494
Test: atest com.android.server.display
Test: atest AutomaticBrightnessControllerTest
Change-Id: I4a53f5ea396c353c69b006dfda76e2f7acce7651