Modified "Empty button" > "Leave empty"

"Empty button" implies a placeholder button, which could be confusing to new users.

Change-Id: I092bdb0c8d3cfad79560526f383cdb7efb730a06

Conflicts:
	packages/SystemUI/res/values/cm_strings.xml
SysUI: Don't recreate controllers.

The various controllers, such as mLocationController, get recreated
every time the status bar is recreated due to a theme change.  We
should only need to create these if they are null, otherwise use
the controller as is.  Also, each of these controlers registers
themselves as a broadcast receiver and are never unregistered before
recreating them.

Change-Id: I6215aae5a82b7ced8f0f25cf592a804d38825260

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/phone/PhoneStatusBar.java
Revert "Lockscreen wallpaper for themes"
New theme engine is no longer compatible to build with lockscreen wallpaper

Change-Id: I4e5bc4c85908f9ab56a04db062a7e782ea82b094