SystemUI: allow devices to use a custom torch sysfs path instead of camera iface

RAW initial commit:

Android Lollipop merged the flashlight into SystemUI,
forcing us to use the camera interface, with this patch
we can specify a custom sysfs path to write to it directly,
without using the camera interface at all, it's faster than
using the camera, plus some devices can't open stock camera2 app.
We can configure to use or not the camera interface,
the sysfs path and default brightness value.

Change-Id: I7809403425ed580cbe7ee39e35b43ce082d1f44e