WallpaperManager: rather return a black bitmap from getDefaultWallpaperLocked than null

Change-Id: I686907d1f056012e1b10a5538e58634e9301d48a
(cherry picked from commit e80d2a92317038cb1b6b12909fffc7ce698958de)
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
(cherry picked from commit 676364a1788d4c87459ebb6e578238d68ed093a6)