Check whether we can scroll vertically in the drag direction

[dtvc87]

I'm not familiar with the internals and I'm not sure if this has side-effects, I found this problem while testing MotionLayout. In all the examples, scrolling up triggers a layout transition, but I noticed it is not possible to trigger a layout transition scrolling down. Looks likeMotionLayout is always checking if we can scroll upwards, so if we scroll to the bottom of a RecyclerView the transition will never be triggered because target.canScrollVertically(-1) will always be true.

Here you can see a video with the fix:
https://github.com/dtvc87/constraintlayout/blob/drag_video/constraintlayout/device-2021-03-10-151947.webm
The red RecylerView pulls down the blue one and the blue RecylerView pushes up the red one, once all its scroll is consumed.

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[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

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[dtvc87]

@jafu888 @camaelon thank you for merging this! :) . I hope I can see you again at droidkaigi in Tokyo when the pandemic is over!