WallpaperCropper: handle decoding errors for preview generation

Repro:
 - Open wallpaper picker
 - Select "Pick image"
 - In DocumentsUI, open drawer and select "Photos" app
 - Select incompatible file (e.g. video, gif)
 - Observe: Force Close

Change-Id: Ibd873ddfd631a4bd8ba53fc22c7d543e6702cd00