Bug 1068967 - Make thumbnails accessible.

It might be helpful to move this code inside MediaUtils similar to formatDate - shared/js/media/media_utils.js , if we are going to have similar changes to use above alt values at other places in media apps e.g. fullscreen view, image editor, Video app list view, 

Also , alt tag with value e.g. 'Photo taken on 9:14 AM, November 20, 2014' is rendering in gray thumbnails grid on first time load just before the image rendering, can we make alt text invisible by using alt text color same as img background.