sensors: Add workaround for Foxconn's broken sensor data

FIH has a tendency to add in-kernel "shortcuts" for stuff that's usually
controlled in userspace, and disabling the screen+backlight on proximity
was one of those. That implementation is, however, buggy, and on top of that
it's colliding with the standard userspace implementation.

This should be corrected at the sensors HAL, but reimplementing that
has been a work-in-progress for quite some time now and still isn't functional.
So, for now, toggle a workaround with TARGET_HAS_FOXCONN_SENSORS, and
convert the data into the kind of values the framework expects.

Change-Id: Ib974af729ce0511668c835b9078831330b874a1f