[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

Conflicts:
	core/java/android/provider/Settings.java
	packages/Keyguard/src/com/android/keyguard/KeyguardViewManager.java