Reparent remaining tasks to matching launch root when display removed

When display removed, reparent remaining tasks to the top of matching
launch root instead of having special logic for reparenting task to
split root. And update focus to the new root task if the last reparented
task has been reparented to another root task.

Bug: 177190100
Test: Passed existing tests
Change-Id: I5c0f4c52a01221bb89001ebc0ca25319ae41b8dd