Add stub method isLockscreenLiveWallpaperEnabled() in qpr

Starting from U, this method returns a config boolean and serves as a flag to implement the new wallpaper logic.

In T, it is just a stub method that always return false.

Bug: 197814683
Test: treehugger (low-risk, no logic changes)
Change-Id: Iac70488c339629a474ad3bd50ae220dc18992e3d
Merged-In: I791ba72f571f996906d2c23f97bba7b88b2fe21f