bug(drag-drop): Auto-scroll can break while dragging between drop lists

Auto-scroll functionality can break if an item is dropped in a list while auto-scrolling, then attempting to perform that same auto-scroll sequence.

[ng-run demo](https://ng-run.com/edit/m0ciJ24IuzdCDaV31wVR)

1) Drag an item from one list to another while auto-scrolling.
2) While auto-scroll is still taking place (this is important), drop the item into the new list.
3) Scroll back to the original list and try the exact same sequence again with another item.

Auto-scrolling will stop once the item has entered the second drop list.

GIF demonstration:


***Problem***

Auto-scroll properties (`_scrollNode`, `_verticalScrollDirection`, `horizontalScrollDirection`) are not being cleared when auto-scroll is stopped.

Upon the second auto-scroll sequence entering the bottom drop list, the following conditional statement (noted with the `HERE` comment) returns false because all the old (non-cleared) properties are the same as the current ones...yet auto-scroll should still be triggered.

```ts
/**
* Checks whether the user's pointer is close to the edges of either the
* viewport or the drop list and starts the auto-scroll sequence.
* @param pointerX User's pointer position along the x axis.
* @param pointerY User's pointer position along the y axis.
*/
_startScrollingIfNecessary(pointerX: number, pointerY: number) {
if (this.autoScrollDisabled) {
return;
}

let scrollNode: HTMLElement | Window | undefined;
let verticalScrollDirection = AutoScrollVerticalDirection.NONE;
let horizontalScrollDirection = AutoScrollHorizontalDirection.NONE;

//...

// <-- HERE-->
if (scrollNode && (verticalScrollDirection !== this._verticalScrollDirection ||
horizontalScrollDirection !== this._horizontalScrollDirection ||
scrollNode !== this._scrollNode)) {
this._verticalScrollDirection = verticalScrollDirection;
this._horizontalScrollDirection = horizontalScrollDirection;
this._scrollNode = scrollNode;

if ((verticalScrollDirection || horizontalScrollDirection) && scrollNode) {
this._ngZone.runOutsideAngular(this._startScrollInterval);
} else {
this._stopScrolling();
}
}
}
```

#### Environment

- Angular:
- CDK/Material: 9.2.2
- Browser(s):
- Operating System (e.g. Windows, macOS, Ubuntu):

Start at the _startScrollingIfNecessary method shown in the issue and trace its interaction with _stopScrolling and the auto-scroll state properties. Reproduce the sequence in the linked ng-run demo, then verify that repeating the drag-and-drop sequence restarts auto-scrolling after an item is dropped during scrolling.

Written by the indexing model from the issue text.