Add interfaces to abstract out Shell DragAndDrop behavior

* No logical changes. Only non interface change is now the
rootView in DragAndDropController is passed to drag layout
to add what it needs instead of assuming that DragLayout is both
the controller and the view

Bug: 349828130
Test: Works same w/ flag on or off
Flag: com.android.wm.shell.enable_flexible_split
Change-Id: Iae870dcc56f9e4fe2c7b8224b1eb1491279216c1