Only wrap leaf tasks when creating legacy remote targets

Legacy animations only know how to deal with leaf tasks,
so don't give them any non-leaf tasks. We were originally trying
to hack around this by rewriting task-ids and such, but that
seemed a bit fragile and there were likely several places where
that change was still missing.

This CL just switches back to all leafs.

Bug: 228883780
Test: use split-screen in shell-transitions.
Change-Id: Ic18412ea9e20e1b2f133a936c63d1a6576ce50a6