Recent folders on tablet in conversation view only.

Fix b/5892452

There are two changes in this:

1. Allow suppression of recent folders, the header and the "Show all
   labels" footer.  2. Introduce a dead header to satisfy
   actionBar.setSelectedNavigationItem, so that we can receive taps on
   every visible account and folder. It is an empty, invisible view
   that is shown above all the accounts. The user cannot tap on it,
   and the user cannot see it. I'm not proud of this hack. We should
   request the framework team to add a setNavigationNone for such
   use-cases.

Change-Id: I83ec4bbb9bbac02941bb5d43a123a65e0afcb7f9
4 files changed