persist the changed position values in updateWallpaperOffsetLocked, even with a null mWallpaperTarget

Change-Id: I175773f6e0b3f91c1d983857028a89b959e4e9f2
Merge commit 'f3e12ab25b52bb4b0f15b9e6088aabfb5ebcae85' into android-4.4