Fix 2091: Respect "Fast Card Browser Open" option in drop-down menu

Fixes [2091](https://code.google.com/p/ankidroid/issues/detail?id=2091).

When I first implemented the drop-down menu I didn't realize that `setSelectedNavigationItem(0, 0)` is automatically called at some point after `onResume` by the system. This behavior circumvented our *Fast Card Browser Open* check. Therefore, I had to remove the call to `onSearch` in `onCreate` and move it into `onNavigationItemSelected` to support that option. I don't really like the fact that the initial search is initiated by the drop-down menu, but I currently don't see a way around it.

I also implemented `OnNavigationListener` to make the `onNavigationItemSelected` option more prominent.