Make AppSearchBar more flexible

[dfernandezm]

Right now this search bar is just a text field. However, in the future we may have many more
screens which need searching, so it would be great if AppSearchBar abstracts away the logic
of switching the app bars based on the state internally so that the top-level screens don't
have to know about it and just use it out-of-the-box.

This implementation can then expose the properties or provide defaults as appropriate.