Allowing leaf Task to contain both Activity and TaskFragment

Allowing a leaf Task to both contains ActivityRecord and
TaskFragment as the direct children.

For example:
   Task
      - TaskFragment
         - Activity C
      - TaskFragment
         - Activity B
      - Activity A

Bug: 189385903
Test: wm presubmit
Change-Id: I79667df70dcbbefbf5a5cce1a24165247f403e69