Move ImageWallpaper to SystemUI process.

Nice to not load 4MB bitmaps in the system process.

Also, hey, with how we are now scrolling the surface instead of
the bitmap, there is no reason to keep that 4MB bitmap loaded in
to memory.  So don't.

Unfortunately it looks like for some reason the VM is still
holding on to the bitmap.  I'll need to figure out why.  Later.

Change-Id: Ib3503756144502fc5c8d5e294248c2417c4fe8c8