Adjust caption observer for desktop transition

Instead of specific transition types, just try to get captions
and if it doesn't exist, make one. As we add more "source"
windowing modes, we'll have to just check more listeners. This
should hold-over until there's a centralized task-state manager.

Also, try to create captions on "warm-starts" (TO_FRONT) so that
apps that are in history before desktop-mode will get a caption.

Bug: 246851689
Bug: 246637353
Test: enable shell-transitions and switch into/out-of desktop
      mode with varying sets of warm/cold-start apps
Change-Id: If9e1ad35692b63bf8abd6185b037715b285645c9