Fix Walleye dim brightness curves

This fixes the issue that cause -1.0f to be added to the brightness
curves when a devices had 0 in the backlight configuration
(config_screenBrightnessBacklight) in config.xml.

This code is the same as it originally was before the float brightness changes.

Bug: 153290107

Test: manual, check values in `adb shell dumpsys display | grep -i
"spline\|mconfig\|mambientlux"`

Change-Id: I332fc09565b26bfc5f108fe32bf109f9584c60a2