Determine split leash position by taskId instead of bounds

* Previously we were comparing bounds of the leash to determine
which leash was leftTop vs bottomRight
* That didn't work when we were translating the split apps when
IME is showing in portrait split screen
* Transitions between IME showing to recents animation could
probably use some work, but that itself is not a regression.

Bug: 330714602
Test: Tested on large and small screen with IME up on both apps
No more empty hole

Change-Id: Ie08ac644e5d1bcc48ba57e05e25bbee66afa3d37