[Mobile, accessibility] Account selector widget doesn't indicate that it's a selector in iPhone VoiceOver mode

Breaking out from http://status.net/open-source/issues/2782

The account selector widget can be selected by touch in iPhone's VoiceOver accessibility mode, but only reads out the account name. It should indicate that it can be activated, and give some clue that activating it will open the account list dialog.

Using a button may help, but I'm not sure we can squeeze in additional voice-only labels to indicate that it's a chooser.