Fix to memory leak leaving WallpaperCropActivity.

After running wallpaper cropper serveral times it crashes with
an OutOfMemotyException.

When leaving WallpaperCropActivity all textures are leaking until end
of process lifetime. This is due to a missing call to function
TiledImageView.destroy which in turn assures TiledImageRenderer
releases held resources.

Resolved by assuring WallpaperCropActivity calls TiledImageView.destroy
from WallpaperCropActivity.onDestroy. In case where onDestroy is not
called resources will be released anyway by process being brought down.

Change-Id: I016b1b9da5f4db8d2b14767edb59fe742415e074