Add flagged drag to desktop transition handler

Setup
- Converted `DragToDesktopTransitionHandler` to sealed class and added 2
  subclasses with default and flagged spring implementation.
- Flagged spring class will override/implement some of the transition handler
  behavior.
- `WMShellModule` provides default or flagged transition handler based
  on `enable_desktop_windowing_transitions` flag value

Flagged changes
- Spring transition handler changes the start drag to desktop
  transition layering and hides home leash on transition end
- It's also going to be used to update the drag to desktop animations in the following CL

Bug: 331163734
Test: atest WMShellUnitTests:DragToDesktopTransitionHandlerTest
Flag: com.android.window.flags.enable_desktop_windowing_transitions
Change-Id: Ia73e29163735c4feb4116fd9b1ad27ff4b247542