Add basic implementation of starting an intent on an unhandled drag

- Expose drags through DragAndDropController, DesktopModeController will
  listen for unhandled drag events and consume it to start the associated
  task

Bug: 320797628
Test: atest WMShellUnitTests
Change-Id: I9d114f58a3224299f3ccb060543533b6fbf4c9ff