This makes "Inbox" the top level with the mailbox list one level under.
All "up"/"back" navigation is now identical and will take you upwards
towards the inbox before falling off to the launcher.
This uses a kludge and the "backstack" of the activity is not actually
represented now by the mPreviousFragment variable; it just uses tricker
in the onBackPressed()/shouldPopFromBackStack() calls to do the right
thing.
Bug: 5111629
Change-Id: Ide88efc78fe4f40d89b5ab83c480318e81902554
e91ff527ed