NULL previous background bitmap upon changing wallpaper

Step to reproduce it on Nexus 10 with 4.4.2(KOT49H):
1. Long press on home screen.
2. Choose wallpaper from Wallpapers.
3. Select new wallpaper and set it.
4. Repeat step 1-3 several times.
See black background instead of the wallpaper.

There are two binder objects who hold reference to the
ImageWallpaper$DrawableEngine, which keeps the big chunk
bitmap from being recycled.
One is WallpaperService$IWallpaperEngineWrapper. The client
references went away slowly, maybe several minutes after
changing wallpaper. Then the finalizer has to been executed
to GC it.
The other one is WallpaperService$Engine$BaseIWindow. Don't
know who still held reference to it even after the window
was removed.
Anyway, let the bitmap be GCed first.

Change-Id: I27f6971a3edd26472b69e59b542b27fd7c8e7b90
Signed-off-by: jshe32X<jianchunx.shen@intel.com>
Signed-off-by: Guobin Zhang <guobin.zhang@intel.com>