Have createRootTask return TaskAppearInfo

We return TaskAppearInfo on TaskOrganizer#createRootTask so it doesn't
have to wait for taskAppeared signal to take action.

Also, added createRootTask(..., listener) method to ShellTaskOrganizer
so that a specific listener can be associated with a root task we are
creating.

Bug: 169266958
Test: split-screen works.
Change-Id: I5948e623481deadb0aa5fc99b9941914f44a68e2