Migrate unfold animation to Shell transitions [Part 1]

Adds a trigger to DisplayContent that creates
unfold transition before applying the display size
and marks it as ready when the sizes are applied.

Adds a placeholder animation for fullscreen tasks.

Bug: 204925795
Test: enable shell transitions, open an app, unfold =>
 check that app surface is animated
Change-Id: Ia88178a7c7849a99c17b4d18117a05b2a0fb8b7f