Avoid any NPE in ImageWallpaper#onDestroy

In rare cases where WallpaperService#doAttachEngine throws a
RemoteException, it will call ImageWallpaper#onDestroy in a way it can
cause a NPE.

Add null checks to avoid that.

Flag: NA
Bug: 330945783
Test: treehugger
Change-Id: Icd8ff54fc765102f50fb543ab8cc4d1d4d5295fc
1 file changed