Make browser search cancellable

This removes the fast browser open preference in favor of making the search operation itself cancellable. This is more intuitive, and makes the new collapsible search view feel much more natural to use with a large collection. The implementation itself is a bit hacky due to the way we handle queued DeckTasks, but I've commented these hacky parts properly. In the long term I think we need to completely move away from using `AsyncTask`s, but that would involve quite a lot of rewriting, so just something else for the TODO list.

Thanks to @chajadan for the idea

Note that I also threw in a trivial fix as a separate commit here which fixes a bug that made the add note button appear twice on tablets.