[1/2] Base: Lockscreen wallpaper

Allows user to choose a custom wallpaper for lockscreen
Also allows users to set a color instead of wallpaper

OmniGears: https://gerrit.omnirom.org/#/c/6374/

Change-Id: I97a6d31ef8915bc3a12b48426afbd6d0b92a41f6
[1/2] Base: Lockscreen wallpaper

Allows user to choose a custom wallpaper for lockscreen
Also allows users to set a color instead of wallpaper
OmniGears: https://gerrit.omnirom.org/#/c/6374/