Fix placeholder not shown after launch again

This issue is regressed by [1].
When we verify if the container is top active container,
we should filter out the case that the Activity is not in
TaskFragmentContainer.

fixes: 224431790
Test: manual - test steps mentioned in bug

[1]: 3d419be7

Change-Id: Iae3b2b903bf15448d429b8edd25ceeef97e74738