Calculate and provide more info necessary for animations

This adds relative offsets so that animating surfaces can
be reparented with the knowledge of how to position them
within their new parents.

This also creates a "rootleash" which serves as a place for the
transition player to put animating surfaces into where they
can all be siblings. This provides control over z-ordering
during the animation and removes relative cropping
constraints. This rootleash surface is a child of the
deepest container that is an ancestor to all animating
targets. It's layer is picked to be the top-most(z) of the
ancestor's direct children which contains animating
targets.

This then Updates the example transition in shell to use
the rootleash.

Bug: 169035082
Test: atest TransitionTests
Change-Id: If40872138cbb6087b46be4a93c298bad94e88ca1