Prevent setting the launch task id when docking.

- In this case, recents is the last running task instead of the 
  previously fullscreen app (likely due to some preparation for the 
  docking), and we should not set the task to launch/cancel window 
  transitions for.

Bug: 28943422
Change-Id: I3c381d3fa33a69adb7d8b606633433a898841526