[1/2] Base: Lockscreen wallpaper

Allows user to choose a custom wallpaper for lockscreen
Changeable through Settings->Display->Wallpaper->Lockscreen
Or Settings->SecuritySettings->Lockscreen->CustomBackground
OmniGears: https://gerrit.omnirom.org/6574
Change-Id: I97a6d31ef8915bc3a12b48426afbd6d0b92a41f6
Patch #26 from OmniRom