Fix moveToBack behavior in organizer-created root tasks

Now, while moving a task and its parents to back, if it
runs into a createdbyorganizer task, it will stop there and
reparent itself into whatever container the home-stack is
in.

Bug: 153081856
Test: added tests to ActivityStackTests
Change-Id: I0be24bd2e753a58bea9bf00724bc4131836b6011