Fix first drag to split not working

There's a timing when dragging to split opening a new app, the new app
might not be put into DC.mOpeningApps yet while startLegacyTransition
triggering app transition. And result to executing an extra TRANSIT_NONE
app transition. This patch leaves the app transition triggering to
ActivityTaskSupervisor.reportResumedActivityLocked where the new opening
app has been put into DC.mOpeningApps.

With CL[1], it'll invoke onAnimationCancelled when there's no valid app
to anmate while executing app transition. So we don't need to cancel
animation with startActivity result.

Also unbind exit split screen logic with onAnimationCancelled in
wm-shell since it only enters split screen after a new task being
launched/reparented to the side stage root after CL[2].

[1]: I50ddc91f7bd7ed24a977fa3face9c6e56e1bea02
[2]: I1cad96f6265eb1a14369bfc708871f701764066a

Fix: 197917994
Test: drag and drop to split works
Change-Id: If9ab7c3a89c8153c3731103571162fe21c94b3aa