Extend recents mixer to handle finishing with a bookend transition

- Currently, splitscreen will listen for the recents transition to start
  (to include the divider in the transition) and end (to restore the
  divider visibility if returning to the app).  It is notified of the
  recents events by RecentsMixedTransition, which wraps the finish
  callback, and passes the existing finishWCT (for the transition that
  starts the recents transition), which split will then query for changes
  that reorder the split stages to the top.

  However, when the recents bookend transitions are enabled, the
  restoring of the transient order is done in a separate transition,
  and the existing logic of looking at the finishWCT fails.

  Instead, we can extend the RecentsMixedHandler callbacks to allow
  RecentsMixedTransition to be notified when a finish is about to
  happen with direct information about whether the finish will return
  to the app.  Using this, split can then modify the finishWCT to
  include any necessary cleanup.

Bug: 346588978
Bug: 391486839
Flag: com.android.wm.shell.enable_recents_bookend_transition
Test: atest WMShellUnitTests
Change-Id: Ifebd4fbde6c7871832309b362553286f017bf2c7