Prevent the launching activity be reparented into embedded task

- Fixing the issue by ignoring the activities in the embedded task
  while searching for the task top activity.

- Also making the embedded tasks to be the candidate while
  looking for a reusable task for the launching activity.

- Updated Task#getOccludingActivityAbove that the activity
  on the same TaskFragment or the activity bounds occludes
  all of the parents up to a non-embedded leaf task.

Bug: 194315692
Test: start activity while another task embedded

Change-Id: Ib8c435ce5a212070d9fe1711c539501d394c33ae