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

...t load up images/blank
I'd suggest make `_triggerActivity` return a promise and then we can limit the access to `this._pickPending` in the scope of `selectWallpaper`. As we are actually ignoring the pending pick, let's rename the property as `this._selectingWallpaper`.

And it would be easier for unit testing if you also make `selectWallpaper return a promise.