App Pairs: Implement Taskbar functionality

[App Pairs 8/?]

This CL adds taskbar functionality for app pairs:
- Ability to drag an app pair icon to Taskbar
- App pair launch from Home Taskbar
- App pair launch from Overview Taskbar
- App pair launch from in-app Taskbar

KNOWN ISSUES:
- Bug (b/315190686): if user is inside a running split pair and attempts to launch the same pair from an app pair icon on taskbar, Overview tiles get temporarily messed up (recoverable by leaving Overview)
- User can attempt to split with an app pair icon on the Taskbar. This should result in a "can't split with this" bounce animation on the SplitInstructionsView (to be implemented). Currently does nothing.

Bug: 274835596
Flag: ACONFIG com.android.wm.shell.enable_app_pairs DEVELOPMENT
Test: Manual
Change-Id: I5256547af236fc2deeb192d60bfe1f2b7ddc5647