Support PER_DISPLAY_BRIGHTNESS.

For MD target PER_DISPLAY_BRIGHTNESS must be supported. For non-md
target, PER_DISPLAY_BRIGHTNESS is also supported but optional. Because
we only have one reference config, we support PER_DISPLAY_BRIGHTNESS.

In reality, this property is doing nothing other than storing the value
into memory map.

Flag: EXEMPT HAL change
Test: Manual test on gcar_md
Bug: 356419293
Change-Id: I7c5c412bd58b25478336c82a47ea2ee0b774335e