Do not reposition leaf tasks from grandparent tasks

A leaf task was added to be the child of its grandparent while
switch user, which caused exception while reparenting split-screen
tasks to fullscreen.

Bug: 161167885
Test: switch user in split-screen
Test: atest TaskTests
Change-Id: Iffde4ddfbe315d0226961f7d98a897b34062c376
Merged-In: Iffde4ddfbe315d0226961f7d98a897b34062c376