Migrate unfold animation to Shell transitions [Part 3]

Adds real unfold animation for split-screen tasks when
doing the Shell unfold transition.
The approach is similar to full-screen tasks:
we animate the surfaces using shell transition
only when unfolding, when folding we are doing
it in the old way (by directly accessing
the surfaces from TaskOrganizer).

Refactored the previous fullscreen/splitscreen unfold
controllers flow to have one controller where we can
register diferrent 'animators'. This controller listens
to all task events in the shell task organizer.

Bug: 204925795
Test: folding/unfolding, rotating with a split layout
  when Shell transitions enabled/disabled,
  rotating display when folded/unfolded
Change-Id: I7bd398c963125ceb7c8f8e3786a274853366feb3