Fix "Previous slideshow preview can't be cleaned"

It maybe a ImageView bug that the resource in ImageView
cannot set to NULL by invoking setImageUri(null).

So the preview in SlideshowAttachmentView will be displayed
as the previous one until Slideshow model has changed.

Change-Id: Ib64318a58c8304ce9f84043a9429f15a5b3f5710
Signed-off-by: Roger Chen <cxr514033970@gmail.com>