Add support for user actions in overlays

This CL adds new user actions that can be used to animate overlays in &
out using swipe gestures:

 - UserActionResult.ShowOverlay
 - UserActionResult.HideOverlay
 - UserActionResult.ReplaceByOverlay

We now use the overlay with the highest zIndex (if any) when fetching
the current user actions.

Bug: 353679003
Test: atest DraggableHandlerTest
Flag: com.android.systemui.scene_container

Change-Id: Ibaa2dfc9a172cbc75cce9afb095be836a64f0f0c