Touchkey light duration preference.  Needs overlay.

Needs an overlay added in the device source and will only work
if your touchkey driver supports this function.  Overlay goes
in the settings/res/values/config.xml file as follows:
<bool name="config_show_touchKeyDur">true</bool>

Change-Id: If5b4710df9fc1c4f181627d496f1ee08254cf6b7
Signed-off-by: scotthartbti <scott@ubuntu.com>