Find the 5.1 source directory: frameworks\base\packages\SettingsProvider\res\values\defaults.xml
Change the value of the def_lockscreen_disabled attribute to true, recompile and install
Find the 5.1 source directory: frameworks\base\packages\SettingsProvider\res\values\defaults.xml
Change the value of the def_lockscreen_disabled attribute to true, recompile and install