Fix problem where wrong colors would be picked

getWallpaperSafeLocked doesn't do what we need, it creates a new
WallpaperData if a lock screen wallpaper isn't set.
The desired behavior would be to get the current system wallpaper colors instead.

There was also a synchronization problem in there.

Test: set live wallpaper and look at lock screen colors
Change-Id: I12a2acb6956ba31bb1d3c0843e04b8e4566e4f12