Fix accessibility issue for search fragment buttons

ToC and overflow menu didn't have any contentDescription,
so TalkBack could not pick up the meaning for them. This helps
visually impaired people to better use our app.

https://ticket.wikimedia.org/otrs/index.pl?Action=AgentZoom&TicketID=7609290

Change-Id: I8f1cac67e98d3cf0255da92c23af75f0c6b1cfdf