SystemUI: Bring back lockscreen tuner (1/2)

* Enables us to change lockscreen shortcuts, but they will be disabled as default.
  (Leaving SystemUI/config.xml as it is makes this possible.)

* Adds Voice Assist and Camera shortcuts to tuner and replaces "None" shortcut
  with real "None". Until now, "None" was either "None" or default shortcut, depending
  on "config_keyguardShowBlablaAffordance".

* It will be located under Settings > Security & Location > Lock screen preferences

Change-Id: Ibb107e550ff492219fb6fedd63f8fc8ee89753a9
Signed-off-by: eray orçunus <erayorcunus@gmail.com>