SurfaceFlinger: Allow forcing HWC brightness support

Some vendors purposely disable brightness capability
even though the HWC completely supports it, in order
to leverage their custom brightness implementations.
One such example of this is in stock HWC of some
xiaomi devices.

Since light HAL offers lesser granual control than
HWC (since its in gamma space and not float), and
since it would be rather difficult to patch the blob
to return the right capabilities, allow forcing the
usage of HWC brightness functions via a prop:

ro.sf.force_hwc_brightness=1 (default 0).

Change-Id: I7d2fef03afbbb936572c717f769f30c9e49213e0
Signed-off-by: Adithya R <gh0strider.2k18.reborn@gmail.com>
Signed-off-by: Simão Gomes Viana <devel@superboring.dev>