Ignore touches in taskbar and all apps windows during system drag.

Touches are ignored as soon as we want to start system drag so that system drag can start sooner (i.e. before any AbstractFloatingView animations finish). This approach utilizes ViewTreeObserverWrapper's compute insets listener by temporarily setting the touch region to empty. The taskbar window remains fullscreen until the drag finishes so the touch region is reset at the right point. Similarly, the all apps window is kept open during its drag operations until the drag finishes. System drag state is now exposed through the drag controller to skip predrag.

Test: Manual by dragging to split screen and triggering dismissal
animation from both windows. Verified predrag works.
Fix: 221104066
Fix: 220070070

Change-Id: I424106269c841f58cbe5338d30b6c33fbd889019