Fix Pair-to-Pair animation being canceled in recents transition handler.

The recents animation should be canceled when leaft task changed(e.g.
expand Pip), so there should ignore structural change from non-leaf
tasks.

Bug: 286024881
Test: Put app in Pip, open another app, go to recents, expand pip
Test: Verify animation won't break when playing Pair-to-Pair switch.
Test: Do above tests with 3p Launcher.
Change-Id: I5d0f2c5750d0cad3887ed95f268cb42bd5961fdb