Decouple DesktopTasksController from DesktopModeWindowDecorViewModel.

DesktopController shouldn't be aware of the ViewModel's existence, and it
shouldn't initiate tiling directly.

This CL introduces a new interface between DesktopTasksController and
DesktopModeWindowDecorView model, named SnapEventHandler, which allows
the controller to communicate the snapping events to the view model without
any exposure.
Flag: com.android.window.flags.enable_tile_resizing

Test: unit tests and on device testing

Bug: 374309287

Change-Id: I1025c4dec7aeb60ea5c008edadac9ba88bfd438b