Some tweaks to improve document task creation.

- Mark the chooser activity as never launching in to a new
  task, even if the caller asks for it.  These are dialogs
  so don't make sense as stand-alone tasks.  (Maybe later
  the policy should be to not launch into a new task in any
  case that the activity is a dialog or even transparent at all.)

- Keep track in the task record of whether any activities in
  it have been shown to the user, and use this to automatically
  remove the task when all activities finish.  This leans up
  cases where apps are launching stub activities that get turned
  in to tasks but are never seen by the user because they
  immediately launch another activity in another task and
  then finish.

Change-Id: I00b641c80aa96bd5071479f36ee2e4d8e3a81aeb