Migrate unfold animation to Shell transitions [Part 2]

Adds real unfold animation for full-screen tasks when
doing the Shell unfold transition. When folding
the animation is done using the previous approach:
by listening TaskOrganizer and modifying the surfaces
without actually starting Shell transitions.

Common logic that handles surface modifications is
extracted to FullscreenUnfoldTaskAnimator.

Bug: 204925795
Test: folding/unfolding with a fullscreen app
  when Shell transitions enabled/disabled
Change-Id: I6d086b4df84d37ae61a46956bc99f0c925aa1755