Fix regression in getting top running task to split with

- ag/28156918 introduced a regression where we were not taking the
  top task to split with (the call into WM returns the list
  most -> least recent)

Fixes: 352676850
Bug: 350016003
Flag: EXEMPT bugfix
Test: Trigger split with a multi-instance app
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:f692ae208d6a98f71d9368812664a046300045cc)
Merged-In: Ib2fd620ae665ef30252abf6e072d9bee8b7ae060
Change-Id: Ib2fd620ae665ef30252abf6e072d9bee8b7ae060