Add support/adapter for recents animation via shell transition

This does the basics to adapt recents animation to shell transition.

Replaces startRecentsActivity with a normal startActivity. Once
we make a full switch to shell transitions, we can hopefully remove
it.

Updates the TransitionHandlers in shell and RemoteTransitions to
support attaching a WindowContainerTransaction to a transition's
finish. For recents, this is used to move the "pausing" app (the
one getting animated into the recents carousel) back to the front
if the gesture doesn't go all-the-way into recents.

Bug: 162503077
Test: enable shell transit. launch an app, use gesture to open
      recents. use gesture to start going to recents, but then
      release early to return to the app.
      atest ShellTransitionTests
Change-Id: Ied54f1b2c70c5c129b2f5798117d6d8a85360998