This introduces tab navigation on large screens with action bar (a
dropdown for the phone view is yet to be implemented, though the
internals are ready for it).
This requires the side effect that restoring a draft
reply/replyall/forward will attempt to also load the source message in
full for additional information. If that load fails for whatever reason,
the draft just remains a "compose" as it used to before.
Bug: 3117253
Change-Id: I9cff5ed4a5e9abd1338b6dbde28ceb3e4dc2b761
5b7e434917