Bug 1096507 - [Wallpaper] Repeatedly tapping gallery, the screen does no...

...t load up images/blank
I don't think this needs to be on the prototype. It will never be set. The other wallpaperSrc property is on the prototype, but I think that might have something to do with the use of Observable (which I don't actually understand).  I'd err on the side of keeping it simple and just using this._pickPending without defining it here.

Also, since the public API of this module is selectWallpaper, consider renaming the flag to 'selectionPending' and putting the test for it in selectWallpaper itself rather than in _triggerActivity