WallpaperManager: rather return a black bitmap from getDefaultWallpaperLocked than null

Change-Id: I686907d1f056012e1b10a5538e58634e9301d48a
view: make setWallpaperPosition() oneway

It is possible that live wallpaper can be blocked or busy when
setWallpaperPosition is called from the Launcher.  This can block the
Launcher process during scrolling, creating jank.  One
way to fix this problem is making setWallpaperPostion() oneway so the
launcher process does not wait for live-wallpaper process.  This should
not be problematic since setWallpaperPosition does not return a value.

Change-Id: I2f7883bdfd931fab8ad0459de78baaed3f13b83d