Don't start app transition when there is empty TaskFragment

TaskFragment can be empty for either 1. wait for new launch activity, or
2. last activity just removed.

For 1, we want to wait for the activity launch.
For 2, we want to wait for organizer to remove the TaskFragment.

Bug: 196173550
Test: atest WmTests:AppTransitionControllerTest
      #testTransitionGoodToGoForTaskFragments
Change-Id: I73ac239ef2e5a75d05af6f6ae97e3f2278ebb4c3