Changed createRootTask task to be completely async

Currently some of the information about a task created by createRootTask
is returned with the method and the rest like leash is return in
onTaskAppear. Mixing both async and sync infromation return styles
complicates the mental model for this. So, we will have this be
completely aync for now.

Also, fixed an issue where the windowing mode for organizer created
task was not set correctly before the organizer gets the first task
appear signal.

Bug: 169266958
Test: Split-screen works!
Change-Id: Ia4180d5a42838bf566fbf75f40466dfd8237e0f1