Update recent tasks split pairs from split controller

- Add pairs for the top visible children when both roots are populated,
  and update when either side changes.
- Remove pairs if an activity is finished, or if the user dismisses
  split manually
- Remove pairs for associated tasks if they are made visible in fullscreen

Bug: 202740477
Test: atest WMShellUnitTests
Test: adb shell dumpsys activity service SystemUIService WMShell

Change-Id: I87c632dfd1a014d64ad8e25c9e68ea6ca230744e