[DnD] Pass displayId to DragEvent

To support APIs where drag surface is relinquished to caller
and caller decides the animation, e.g.
TaskbarDragController, DesktopTaskController, they need to
know the displayId where the x, y refers to.

Initially there're 2 alternatives here, to either pass -1,
-1 as x, y to denote event happened on different display, or
modify the API such as GlobalDragListener to also receives
displayId as param. However, as it's beneficial for both
ACTION_DRAG_ENDED and ACTION_DROP to have displayId, we
might as well have it inside DragEvent. This also aligns
with the native implementation where DragEvent also
(extending InputEvent) contains displayId.

Bug: 365512241
Test: atest WmTests:DragDropControllerTests
Flag: EXEMPT new variable is unused
Change-Id: I63b4289e5e11f22c5e8cdc2d4fc0453fb114c10b