[1/2] Base: Lockscreen wallpaper

Allows user to choose a custom wallpaper for lockscreen
Change through
Settings->SecuritySettings->Lockscreen->CustomBackground

Add mechanism to disable lockscreen blur if lockscreen
rotation is enabled

OmniGears: https://gerrit.omnirom.org/6574

PS40:
-use documents as primary chooser
-dont rotate wallpaper
-UI rework

PS41:
-cleanup

PS42:
-more cleanup

PS43:
-fixed multi-user handling

Change-Id: I97a6d31ef8915bc3a12b48426afbd6d0b92a41f6