android: Show document browser back in the menu

Related with commit c7c2b4b54d8ab81150e9b71433970c2f338ed063.

There is nothing wrong with having several activities; in fact Android
development encourages using one activity per use case. After this
patch we have:

* One menu item that opens LibreOffice document browser. It covers the
  use case "open LibreOffice without documents".
* The viewer activity does not have a menu item but it responds to
  open intents triggered by any applications like file explorers,
  including the document browser.

Change-Id: Ie08b96e52d3a6a79aed3354620302705d7689381
Reviewed-on: https://gerrit.libreoffice.org/12108
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>