Display: Brightness: Low power mode scales the brightness to 50 percent.

When the device is in low power mode all changes to brightness will be
scaled to half of the brightness that would be normally set when not in
low power mode.

Change-Id: I03be820ef64c1d5631ded35f7dfc2799e807226b
Signed-off-by: Ruchi Kandoi <kandoiruchi@google.com>