Make Brightness Ramp Speeds Configurable

Allow brightness ramp speeds to differ based on whether the brightness is increasing or decreasing.
They already allow fast / slow ramp rates.

If all 4 values (fast up; fast down; slow up; slow down) are put in the DDC, these are used.
Otherwise, config.xml provides one value for fast and one for slow ramp rates, these are used for both increasing and decreasing values.

Bug: 156247142
Test: manual
Change-Id: If0920328e2bf16e3f8ebe6ea1b07a467e5c4e939