Add shell recents interface

- Add a shell component and interface for Launcher to use to get recent
  tasks including paired splits (pending integration w/ split controller)

Bug: 202740477
Test: atest WMShellUnitTests:com.android.wm.shell.recents.RecentTasksControllerTest

Signed-off-by: Winson Chung <winsonc@google.com>
Change-Id: I34d834061ed2391d89cf56d6a2fd262868707674