This feature will be included in the next major update of the mobile version which is the priority we are currently working on.
In general, why doesn't the toolbar at the top/middle of the screen appear in the Android version of Viewer as it does in the Windows version of Viewer?
Not all features can be implemented the same way as they exist in the desktop version. Yet, we will do our best to include as many important features in the mobile update as possible. One of them is a full-fledged address book compatible with the desktop Viewer's address book.