Move drag event to InputDispatcher (6/n)

This CL provides a way of notifying the dropping window to
DragState. That helps DragState could dispatch the drop event to the
corresponding client of the target window by checking the pointer in
InputDispacher.

Bug: 158242495
Test: atest CrossAppDragAndDropTests DragDropTest
Test: atest WmTests:DragDropControllerTests
Change-Id: I09e213852ab53ae5ac1d3aa45b119f176ed3145a