Keep top fullscreen tasks on top when dismiss splits

A fullscreen task was added on top and exit split-screen mode
while an activity started, but the activity didn't be resumed
because the belonging task was no longer on top after split-screen
tasks were reparented to top.

Bug: 161170237
Test: start emergency dialer when split-screen activated

Change-Id: I44dbe0d2966dd831264ca366f5889d6429d78972