SystemUI: Bring back lockscreen tuner (2/2)

* Enables us to change lockscreen shortcuts, but they will be disabled as default.
  (Leaving SystemUI/config.xml as it is makes this possible.)
* It will be located under Settings > Security & Location > Lock screen preferences

Change-Id: I2c5fe80d1d25c9b20376331ebdad850bdb640eb2
Signed-off-by: eray orçunus <erayorcunus@gmail.com>