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>
Null pointer exception in FileRotator.java

Sometimes a null pointer exception is thrown when examining files
managed by the file rotator.

The logs from the test show that the Wifi state is changed a large
number of times. On every state change, a write operation is
initiated on the file system. This will eventually result in out
of memory and the call to mBasePath.list() in the maybeRotate(...)
method in FileRotator.java will return null so the iteration will
throw a NullPointerException.

Change-Id: I5d5980d9593bc9ec75281169ca27ee591809903f