Make DragAndDropController optional

Some device wants to customize wm shell behavior and remove
DragAndDropController.
This change makes it optional in wm shell, and make it configurable from
config.

Bug: 278981746
Test: TreeHugger passing.
Test: Flip the config value and check if ShellDropTarget is not created.
Change-Id: I44911ec45f2e33fb9500d80054aad10006c98402