sensors: Add dummy light sensor

Some devices have a sysfs toggle for the lightsensor instead of
passing the LUX values for userspace processing; for those,
use BOARD_SYSFS_LIGHT_SENSOR:=<sysfs path>

This is necessary for the rest of userspace to recognize its existence
and toggle it for auto-brightness. It doesn't generate actual light
values, though; all backlight adjustments are made in-kernel.

Change-Id: I0e546b4740720bd34d1e1d85a96b295fcc697106

sensors: dummy ls: set unique handle

handles and types are usually the same, but it's not mandatory.
Sony DASH use custom handles, and the previous assumption broke
Proximity sensor on Sony devices.

Change-Id: I954e13765f42341e6b5393a12556f6a007c4e494
sensordevice: Dummy ALS: Solve fortified libc warning

convert sprint to snprintf, the size of the writes is always known

Change-Id: I53ee36b67544ac25de7e5c05ce90b6817c8a9c92